diff --git a/Build.ps1 b/Build.ps1
index 34a2dc5..dc24499 100644
--- a/Build.ps1
+++ b/Build.ps1
@@ -14,7 +14,7 @@ if($LASTEXITCODE -ne 0) { exit 1 }
 
 $branch = @{ $true = $env:APPVEYOR_REPO_BRANCH; $false = $(git symbolic-ref --short -q HEAD) }[$env:APPVEYOR_REPO_BRANCH -ne $NULL];
 $revision = @{ $true = "{0:00000}" -f [convert]::ToInt32("0" + $env:APPVEYOR_BUILD_NUMBER, 10); $false = "local" }[$env:APPVEYOR_BUILD_NUMBER -ne $NULL];
-$suffix = @{ $true = ""; $false = "$($branch.Substring(0, [math]::Min(10,$branch.Length)))-$revision"}[$branch -eq "master" -and $revision -ne "local"]
+$suffix = @{ $true = ""; $false = "$($branch.Substring(0, [math]::Min(10,$branch.Length)))-$revision"}[$branch -eq "main" -and $revision -ne "local"]
 
 echo "build: Version suffix is $suffix"
 
diff --git a/appveyor.yml b/appveyor.yml
index b7a3eef..26b17bc 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,8 +1,6 @@
 version: '{build}'
 skip_tags: true
-image: Visual Studio 2017
-configuration: Release
-install:
+image: Visual Studio 2022
 build_script:
 - ps: ./Build.ps1
 test: off
@@ -14,7 +12,7 @@ deploy:
     secure: bkES4Ho0Cs/3Ws1PP7fYfSZop6K8VfqAAEFUR25eQRCJXVDPa+atx3alsaMhzVdo
   skip_symbols: true
   on:
-    branch: /^(master|dev)$/
+    branch: /^(main|dev)$/
 - provider: GitHub
   auth_token:
     secure: hX+cZmW+9BCXy7vyH8myWsYdtQHyzzil9K5yvjJv7dK9XmyrGYYDj/DPzMqsXSjo
diff --git a/asset/seq-input-healthcheck-2.png b/asset/seq-input-healthcheck-2.png
deleted file mode 100644
index 9f2fcf4..0000000
Binary files a/asset/seq-input-healthcheck-2.png and /dev/null differ
diff --git a/src/Seq.Input.HealthCheck/JsonDataExtractor.cs b/src/Seq.Input.HealthCheck/Data/JsonDataExtractor.cs
similarity index 77%
rename from src/Seq.Input.HealthCheck/JsonDataExtractor.cs
rename to src/Seq.Input.HealthCheck/Data/JsonDataExtractor.cs
index 73c8260..3927344 100644
--- a/src/Seq.Input.HealthCheck/JsonDataExtractor.cs
+++ b/src/Seq.Input.HealthCheck/Data/JsonDataExtractor.cs
@@ -16,34 +16,33 @@
 using System.IO;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
-using Serilog.Events;
-using Serilog.Filters.Expressions;
+using Serilog.Expressions;
 using Serilog.Formatting.Compact.Reader;
 using Serilog.Formatting.Json;
 
-namespace Seq.Input.HealthCheck
+namespace Seq.Input.HealthCheck.Data
 {
     public class JsonDataExtractor
     {
-        static readonly JsonValueFormatter ValueFormatter = new JsonValueFormatter("$type");
+        static readonly JsonValueFormatter ValueFormatter = new("$type");
         static readonly JsonSerializer Serializer = JsonSerializer.Create(new JsonSerializerSettings
         {
             DateParseHandling = DateParseHandling.None
         });
 
-        readonly Func<JToken, JToken> _extract;
+        readonly Func<JToken?, JToken> _extract;
 
         public JsonDataExtractor(string expression)
         {
             if (expression == "@Properties")
             {
-                _extract = v => v;
+                _extract = v => v ?? JValue.CreateNull();
             }
             else
             {
-                var expr = FilterLanguage.CreateFilter(expression);
+                var expr = SerilogExpression.Compile(expression, nameResolver: new SeqSyntaxNameResolver());
                 _extract = v => {
-                    if (!(v is JObject obj))
+                    if (v is not JObject obj)
                         throw new ArgumentException("Data value extraction requires a JSON object response.");
 
                     if (!obj.ContainsKey("@t"))
@@ -52,16 +51,15 @@ public JsonDataExtractor(string expression)
                     var le = LogEventReader.ReadFromJObject(obj);
 
                     var value = expr(le);
+                    
+                    // `null` here means "undefined", but for most purposes this substitution is convenient.
                     if (value == null)
                         return JValue.CreateNull();
 
-                    if (!(value is LogEventPropertyValue lepv))
-                        return JToken.FromObject(value);
-
                     var sw = new StringWriter();
-                    ValueFormatter.Format(lepv, sw);
+                    ValueFormatter.Format(value, sw);
                     return Serializer.Deserialize<JToken>(
-                        new JsonTextReader(new StringReader(sw.ToString())));
+                        new JsonTextReader(new StringReader(sw.ToString())))!;
                 };
             }
         }
diff --git a/src/Seq.Input.HealthCheck/Data/SeqSyntaxNameResolver.cs b/src/Seq.Input.HealthCheck/Data/SeqSyntaxNameResolver.cs
new file mode 100644
index 0000000..503b81c
--- /dev/null
+++ b/src/Seq.Input.HealthCheck/Data/SeqSyntaxNameResolver.cs
@@ -0,0 +1,46 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+using Serilog.Events;
+using Serilog.Expressions;
+
+namespace Seq.Input.HealthCheck.Data
+{
+    public class SeqSyntaxNameResolver: NameResolver
+    {
+        // ReSharper disable once UnusedMember.Global
+        // ReSharper disable once ReturnTypeCanBeNotNullable
+        public static LogEventPropertyValue? Has(LogEventPropertyValue? value)
+        {
+            return new ScalarValue(value != null);
+        }
+    
+        public override bool TryResolveFunctionName(string name, [NotNullWhen(true)] out MethodInfo? implementation)
+        {
+            if ("Has".Equals(name, StringComparison.OrdinalIgnoreCase))
+            {
+                implementation = GetType().GetMethod("Has", BindingFlags.Static | BindingFlags.Public)!;
+                return true;
+            }
+
+            implementation = null;
+            return false;
+        }
+
+        public override bool TryResolveBuiltInPropertyName(string alias, [NotNullWhen(true)] out string? target)
+        {
+            target = alias switch
+            {
+                "Exception" => "x",
+                "Level" => "l",
+                "Message" => "m",
+                "MessageTemplate" => "mt",
+                "Properties" => "p",
+                "Timestamp" => "t",
+                _ => null
+            };
+        
+            return target != null;
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Seq.Input.HealthCheck/HealthCheckInput.cs b/src/Seq.Input.HealthCheck/HealthCheckInput.cs
index 239bd6f..fe08f39 100644
--- a/src/Seq.Input.HealthCheck/HealthCheckInput.cs
+++ b/src/Seq.Input.HealthCheck/HealthCheckInput.cs
@@ -17,7 +17,11 @@
 using System.IO;
 using System.Net.Http;
 using Seq.Apps;
+using Seq.Input.HealthCheck.Data;
 using Seq.Input.HealthCheck.Util;
+// ReSharper disable MemberCanBePrivate.Global
+// ReSharper disable UnusedAutoPropertyAccessor.Global
+// ReSharper disable AutoPropertyCanBeMadeGetOnly.Global
 
 namespace Seq.Input.HealthCheck
 {
@@ -26,22 +30,22 @@ namespace Seq.Input.HealthCheck
     public class HealthCheckInput : SeqApp, IPublishJson, IDisposable
     {
         readonly List<HealthCheckTask> _healthCheckTasks = new List<HealthCheckTask>();
-        HttpClient _httpClient;
+        HttpClient? _httpClient;
 
         [SeqAppSetting(
             DisplayName = "Target URLs",
             HelpText = "The HTTP or HTTPS URL that the health check will periodically GET. Multiple URLs " +
                        "can be checked; enter one per line.",
             InputType = SettingInputType.LongText)]
-        public string TargetUrl { get; set; }
+        public string TargetUrl { get; set; } = null!;
 
         [SeqAppSetting(InputType = SettingInputType.Password, IsOptional = true, DisplayName = "Authentication Header",
             HelpText = "An optional `Name: Value` header, stored as sensitive data, for authentication purposes.")]
-        public string AuthenticationHeader { get; set; }
+        public string? AuthenticationHeader { get; set; }
         
         [SeqAppSetting(InputType = SettingInputType.LongText, IsOptional = true, DisplayName = "Other Headers",
             HelpText = "Additional headers to send with the request, one per line in `Name: Value` format.")]
-        public string OtherHeaders { get; set; }
+        public string? OtherHeaders { get; set; }
         
         [SeqAppSetting(
             DisplayName = "Bypass HTTP caching",
@@ -64,16 +68,14 @@ public class HealthCheckInput : SeqApp, IPublishJson, IDisposable
                        "The expression will be evaluated against the response to produce a `Data` property" +
                        " on the resulting event. Use the special value `@Properties` to capture the whole " +
                        "response. The response must be UTF-8 `application/json` for this to be applied.")]
-        public string DataExtractionExpression { get; set; }
-
-
-
+        public string? DataExtractionExpression { get; set; }
+        
         public void Start(TextWriter inputWriter)
         {
             _httpClient = HttpHealthCheckClient.Create();
             var reporter = new HealthCheckReporter(inputWriter);
 
-            JsonDataExtractor extractor = null;
+            JsonDataExtractor? extractor = null;
             if (!string.IsNullOrWhiteSpace(DataExtractionExpression))
                 extractor = new JsonDataExtractor(DataExtractionExpression);
 
diff --git a/src/Seq.Input.HealthCheck/HealthCheckResult.cs b/src/Seq.Input.HealthCheck/HealthCheckResult.cs
index c3bf3a2..2296935 100644
--- a/src/Seq.Input.HealthCheck/HealthCheckResult.cs
+++ b/src/Seq.Input.HealthCheck/HealthCheckResult.cs
@@ -16,6 +16,8 @@
 using System.Globalization;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
+// ReSharper disable UnusedAutoPropertyAccessor.Global
+// ReSharper disable MemberCanBePrivate.Global
 
 namespace Seq.Input.HealthCheck
 {
@@ -25,14 +27,14 @@ class HealthCheckResult
         public DateTime UtcTimestamp { get; }
 
         [JsonProperty("@x", DefaultValueHandling = DefaultValueHandling.Ignore)]
-        public string Exception { get; }
+        public string? Exception { get; }
 
         [JsonProperty("@mt")]
         public string MessageTemplate { get; } =
             "Health check {Method} {TargetUrl} {Outcome} with status code {StatusCode} in {Elapsed:0.000} ms";
 
         [JsonProperty("@l", DefaultValueHandling = DefaultValueHandling.Ignore)]
-        public string Level { get; }
+        public string? Level { get; }
 
         [JsonProperty("@r")]
         public string[] Renderings => new[] {Elapsed.ToString("0.000", CultureInfo.InvariantCulture)};
@@ -43,18 +45,18 @@ class HealthCheckResult
         public string Outcome { get; }
         public double Elapsed { get; }
         public int? StatusCode { get; }
-        public string ContentType { get; }
+        public string? ContentType { get; }
         public long? ContentLength { get; }
         public string ProbeId { get; }
 
         [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]
-        public string InitialContent { get; }
+        public string? InitialContent { get; }
 
         [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]
-        public JToken Data { get; }
+        public JToken? Data { get; }
 
         [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]
-        public string ProbedUrl { get; }
+        public string? ProbedUrl { get; }
 
         public HealthCheckResult(
             DateTime utcTimestamp,
@@ -63,15 +65,15 @@ public HealthCheckResult(
             string targetUrl,            
             string outcome,
             string probeId,
-            string level,
+            string? level,
             double elapsed,
             int? statusCode,
-            string contentType,
+            string? contentType,
             long? contentLength,
-            string initialContent,
-            Exception exception,
-            JToken data,
-            string probedUrl)
+            string? initialContent,
+            Exception? exception,
+            JToken? data,
+            string? probedUrl)
         {
             if (utcTimestamp.Kind != DateTimeKind.Utc)
                 throw new ArgumentException("The timestamp must be UTC.", nameof(utcTimestamp));
diff --git a/src/Seq.Input.HealthCheck/HttpHealthCheck.cs b/src/Seq.Input.HealthCheck/HttpHealthCheck.cs
index 88ef7e5..c6e6915 100644
--- a/src/Seq.Input.HealthCheck/HttpHealthCheck.cs
+++ b/src/Seq.Input.HealthCheck/HttpHealthCheck.cs
@@ -22,6 +22,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Newtonsoft.Json.Linq;
+using Seq.Input.HealthCheck.Data;
 using Seq.Input.HealthCheck.Util;
 
 namespace Seq.Input.HealthCheck
@@ -31,18 +32,18 @@ class HttpHealthCheck
         readonly string _title;
         readonly string _targetUrl;
         readonly List<(string, string)> _headers;
-        readonly JsonDataExtractor _extractor;
+        readonly JsonDataExtractor? _extractor;
         readonly bool _bypassHttpCaching;
         readonly HttpClient _httpClient;
         readonly byte[] _buffer = new byte[2048];
 
         public const string ProbeIdParameterName = "__probe";
 
-        static readonly UTF8Encoding ForgivingEncoding = new UTF8Encoding(false, false);
+        static readonly UTF8Encoding ForgivingEncoding = new(false, false);
         const int InitialContentChars = 16;
         const string OutcomeSucceeded = "succeeded", OutcomeFailed = "failed";
 
-        public HttpHealthCheck(HttpClient httpClient, string title, string targetUrl,  List<(string, string)> headers, JsonDataExtractor extractor, bool bypassHttpCaching)
+        public HttpHealthCheck(HttpClient httpClient, string title, string targetUrl,  List<(string, string)> headers, JsonDataExtractor? extractor, bool bypassHttpCaching)
         {
             _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
             _title = title ?? throw new ArgumentNullException(nameof(title));
@@ -56,12 +57,12 @@ public async Task<HealthCheckResult> CheckNow(CancellationToken cancel)
         {
             string outcome;
 
-            Exception exception = null;
+            Exception? exception = null;
             int? statusCode = null;
-            string contentType = null;
+            string? contentType = null;
             long? contentLength = null;
-            string initialContent = null;
-            JToken data = null;
+            string? initialContent = null;
+            JToken? data = null;
 
             var probeId = Nonce.Generate(12);
             var probedUrl = _bypassHttpCaching ?
@@ -75,17 +76,11 @@ public async Task<HealthCheckResult> CheckNow(CancellationToken cancel)
             {
                 var request = new HttpRequestMessage(HttpMethod.Get, probedUrl);
                 request.Headers.Add("X-Correlation-ID", probeId);
-                if (_headers != null)
+
+                foreach (var (name, value) in _headers)
                 {
-                    foreach (var (name, value) in _headers)
-                    {
-                        // the api does not allow overwriting, therefore removal has to happen first.
-                        if (request.Headers.Contains(name))
-                        {
-                            request.Headers.Remove(name);
-                        }
-                        request.Headers.Add(name, value);
-                    }
+                    // This will throw if a header is duplicated (better for the user to detect this configuration problem).
+                    request.Headers.Add(name, value);
                 }
 
                 if (_bypassHttpCaching)
@@ -97,7 +92,7 @@ public async Task<HealthCheckResult> CheckNow(CancellationToken cancel)
                 contentType = response.Content.Headers.ContentType?.ToString();
                 contentLength = response.Content.Headers.ContentLength;
 
-                var content = await response.Content.ReadAsStreamAsync();
+                var content = await response.Content.ReadAsStreamAsync(cancel);
                 (initialContent, data) = await DownloadContent(content, contentType, contentLength);
 
                 outcome = response.IsSuccessStatusCode ? OutcomeSucceeded : OutcomeFailed;
@@ -133,18 +128,18 @@ public async Task<HealthCheckResult> CheckNow(CancellationToken cancel)
         }
 
         // Either initial content, or extracted data
-        async Task<(string initialContent, JToken data)> DownloadContent(Stream body, string contentType, long? contentLength)
+        async Task<(string? initialContent, JToken? data)> DownloadContent(Stream body, string? contentType, long? contentLength)
         {
             if (_extractor == null ||
                 contentLength == 0 ||
                 contentType != "application/json; charset=utf-8" && contentType != "application/json")
             {
-                var read = await body.ReadAsync(_buffer, 0, _buffer.Length);
+                var read = await body.ReadAsync(_buffer);
                 var initial = ForgivingEncoding.GetString(_buffer, 0, Math.Min(read, InitialContentChars));
 
                 // Drain the response to avoid dropped connection errors on the server.
                 while (read > 0)
-                    read = await body.ReadAsync(_buffer, 0, _buffer.Length);
+                    read = await body.ReadAsync(_buffer);
 
                 return (initial, null);
             }
diff --git a/src/Seq.Input.HealthCheck/Properties/AssemblyInfo.cs b/src/Seq.Input.HealthCheck/Properties/AssemblyInfo.cs
deleted file mode 100644
index 07b8df9..0000000
--- a/src/Seq.Input.HealthCheck/Properties/AssemblyInfo.cs
+++ /dev/null
@@ -1,3 +0,0 @@
-using System.Runtime.CompilerServices;
-
-[assembly: InternalsVisibleTo("Seq.Input.HealthCheck.Tests")]
diff --git a/src/Seq.Input.HealthCheck/Seq.Input.HealthCheck.csproj b/src/Seq.Input.HealthCheck/Seq.Input.HealthCheck.csproj
index fb33d54..262635f 100644
--- a/src/Seq.Input.HealthCheck/Seq.Input.HealthCheck.csproj
+++ b/src/Seq.Input.HealthCheck/Seq.Input.HealthCheck.csproj
@@ -1,30 +1,37 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
-    <VersionPrefix>1.2.2</VersionPrefix>
+    <TargetFramework>net5.0</TargetFramework>
+    <VersionPrefix>2.0.0</VersionPrefix>
     <Description>Seq Health Check: periodically GET an HTTP resource and publish response metrics to Seq.</Description>
     <Authors>Datalust and Contributors</Authors>
     <PackageTags>seq-app</PackageTags>
-    <PackageIconUrl>https://raw.githubusercontent.com/datalust/seq-input-healthcheck/dev/asset/seq-input-healthcheck-2.png</PackageIconUrl>
     <PackageProjectUrl>https://github.com/datalust/seq-input-healthcheck</PackageProjectUrl>
     <RepositoryUrl>https://github.com/datalust/seq-input-healthcheck</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <TreatWarningsAsErrors>True</TreatWarningsAsErrors>
     <TreatSpecificWarningsAsErrors />
+    <PackageIcon>seq-input-healthcheck.png</PackageIcon>
     <PackageLicenseFile>LICENSE</PackageLicenseFile>
+    <LangVersion>9</LangVersion>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="Seq.Input.HealthCheck.Tests" />
+  </ItemGroup>
+  
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.WebUtilities" Version="2.2.0" />
-    <PackageReference Include="newtonsoft.json" Version="12.0.1" />
-    <PackageReference Include="Seq.Apps" Version="5.1.0" />
-    <PackageReference Include="Serilog" Version="2.9.0" />
-    <PackageReference Include="Serilog.Filters.Expressions" Version="2.1.0" />
-    <PackageReference Include="Serilog.Formatting.Compact.Reader" Version="1.0.3" />
+    <PackageReference Include="newtonsoft.json" Version="13.0.1" />
+    <PackageReference Include="Seq.Apps" Version="2021.4.0" />
+    <PackageReference Include="Serilog" Version="2.10.0" />
+    <PackageReference Include="Serilog.Expressions" Version="3.3.0" />
+    <PackageReference Include="Serilog.Formatting.Compact.Reader" Version="1.0.5" />
   </ItemGroup>
 
   <ItemGroup>
+    <None Include="../../asset/seq-input-healthcheck.png" Pack="true" Visible="false" PackagePath="" />
     <None Include="../../LICENSE" Pack="true" PackagePath="" />
     <None Include="./obj/publish/**/*" Exclude="./obj/publish/Seq.Input.HealthCheck.dll;./obj/publish/Seq.Apps.dll;./obj/publish/Serilog.dll" Pack="true" PackagePath="lib/$(TargetFramework)" />
   </ItemGroup>
diff --git a/src/Seq.Input.HealthCheck/Util/HeaderSettingFormat.cs b/src/Seq.Input.HealthCheck/Settings/HeaderSettingFormat.cs
similarity index 59%
rename from src/Seq.Input.HealthCheck/Util/HeaderSettingFormat.cs
rename to src/Seq.Input.HealthCheck/Settings/HeaderSettingFormat.cs
index 377a148..b794e6f 100644
--- a/src/Seq.Input.HealthCheck/Util/HeaderSettingFormat.cs
+++ b/src/Seq.Input.HealthCheck/Settings/HeaderSettingFormat.cs
@@ -3,11 +3,9 @@
 using System.IO;
 namespace Seq.Input.HealthCheck.Util
 {
-    // adapted from: https://github.com/datalust/seq-app-httprequest/blob/e23b2328141352f5b4851c7502801f5bfe6511f7/src/Seq.App.HttpRequest/Settings/HeaderSettingFormat.cs
-    // changes to parse for C# 8.0 language level
     static class HeaderSettingFormat
     {
-        public static List<(string, string)> FromSettings(string authenticationHeader, string otherHeaders)
+        public static List<(string, string)> FromSettings(string? authenticationHeader, string? otherHeaders)
         {
             var headers = new List<(string, string)>();
             
@@ -27,16 +25,13 @@ static class HeaderSettingFormat
 
             return headers;
         }
-
+        
         internal static (string, string) Parse(string header)
         {
-            var components = header.Split(new[] { ':' }, 2, StringSplitOptions.RemoveEmptyEntries);
-            if (components.Length != 2)
-            {
-                throw new ArgumentException($"The header must be specified in `Name: Value` format. (Affected line was '{header}')");
-            }
-
-            return (components[0].Trim(), components[1].Trim());
+            var colon = header.IndexOf(":", StringComparison.Ordinal);
+            if (colon is 0 or -1)
+                throw new ArgumentException("The header must be specified in `Name: Value` format.");
+            return (header[..colon].Trim(), header[(colon + 1)..].Trim());
         }
     }
 }
diff --git a/src/Seq.Input.HealthCheck/Util/Nonce.cs b/src/Seq.Input.HealthCheck/Util/Nonce.cs
index 7b68d77..4aa5e18 100644
--- a/src/Seq.Input.HealthCheck/Util/Nonce.cs
+++ b/src/Seq.Input.HealthCheck/Util/Nonce.cs
@@ -31,20 +31,18 @@ static IEnumerable<char> GenerateChars(int count)
         {
             var rem = count;
             var buf = new byte[128];
-            using (var rng = RandomNumberGenerator.Create())
+            using var rng = RandomNumberGenerator.Create();
+            while (rem > 0)
             {
-                while (rem > 0)
+                rng.GetBytes(buf);
+                var b64 = Convert.ToBase64String(buf);
+                for (var i = 0; i < b64.Length && rem > 0; ++i)
                 {
-                    rng.GetBytes(buf);
-                    var b64 = Convert.ToBase64String(buf);
-                    for (var i = 0; i < b64.Length && rem > 0; ++i)
+                    var c = b64[i];
+                    if (char.IsLetterOrDigit(c))
                     {
-                        var c = b64[i];
-                        if (char.IsLetterOrDigit(c))
-                        {
-                            yield return c;
-                            rem--;
-                        }
+                        yield return c;
+                        rem--;
                     }
                 }
             }
diff --git a/test/Seq.Input.HealthCheck.Tests/JsonDataExtractorTests.cs b/test/Seq.Input.HealthCheck.Tests/Data/JsonDataExtractorTests.cs
similarity index 97%
rename from test/Seq.Input.HealthCheck.Tests/JsonDataExtractorTests.cs
rename to test/Seq.Input.HealthCheck.Tests/Data/JsonDataExtractorTests.cs
index eeabdee..eca4e33 100644
--- a/test/Seq.Input.HealthCheck.Tests/JsonDataExtractorTests.cs
+++ b/test/Seq.Input.HealthCheck.Tests/Data/JsonDataExtractorTests.cs
@@ -1,6 +1,7 @@
 using System.IO;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
+using Seq.Input.HealthCheck.Data;
 using Xunit;
 
 namespace Seq.Input.HealthCheck.Tests
diff --git a/test/Seq.Input.HealthCheck.Tests/Seq.Input.HealthCheck.Tests.csproj b/test/Seq.Input.HealthCheck.Tests/Seq.Input.HealthCheck.Tests.csproj
index 3d785ea..103c03e 100644
--- a/test/Seq.Input.HealthCheck.Tests/Seq.Input.HealthCheck.Tests.csproj
+++ b/test/Seq.Input.HealthCheck.Tests/Seq.Input.HealthCheck.Tests.csproj
@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>
diff --git a/test/Seq.Input.HealthCheck.Tests/Settings/HeaderSettingFormatTests.cs b/test/Seq.Input.HealthCheck.Tests/Settings/HeaderSettingFormatTests.cs
new file mode 100644
index 0000000..e9d1126
--- /dev/null
+++ b/test/Seq.Input.HealthCheck.Tests/Settings/HeaderSettingFormatTests.cs
@@ -0,0 +1,34 @@
+using System;
+using Seq.Input.HealthCheck.Util;
+using Xunit;
+
+namespace Seq.Input.HealthCheck.Tests.Settings
+{
+
+    public class HeaderSettingFormatTests
+    {
+        [Theory]
+        [InlineData("")]
+        [InlineData("Name")]
+        [InlineData("Name Value")]
+        [InlineData(":Value")]
+        public void InvalidHeaderSettingsAreNotParsed(string setting)
+        {
+            Assert.Throws<ArgumentException>(() => HeaderSettingFormat.Parse(setting));
+        }
+
+        [Theory]
+        [InlineData("Name: Value", "Name", "Value")]
+        [InlineData("Name:Value", "Name", "Value")]
+        [InlineData("Name: Value ", "Name", "Value")]
+        [InlineData("Name:", "Name", "")]
+        [InlineData("Dashed-Name: Value", "Dashed-Name", "Value")]
+        [InlineData("Name: Value: Value", "Name", "Value: Value")]
+        public void ValidHeaderSettingsAreParsed(string setting, string expectedName, string expectedValue)
+        {
+            var (name, value) = HeaderSettingFormat.Parse(setting);
+            Assert.Equal(expectedName, name);
+            Assert.Equal(expectedValue, value);
+        }
+    }
+}
\ No newline at end of file
diff --git a/test/Seq.Input.HealthCheck.Tests/Util/NonceTests.cs b/test/Seq.Input.HealthCheck.Tests/Util/NonceTests.cs
index 00ee0cd..845e11c 100644
--- a/test/Seq.Input.HealthCheck.Tests/Util/NonceTests.cs
+++ b/test/Seq.Input.HealthCheck.Tests/Util/NonceTests.cs
@@ -7,7 +7,7 @@ namespace Seq.Input.HealthCheck.Tests.Util
 {
     public class NonceTests
     {
-        public static IEnumerable<object[]> CharacterCounts = Enumerable.Range(0, 100).Select(n => new [] { (object)n });
+        public static readonly IEnumerable<object[]> CharacterCounts = Enumerable.Range(0, 100).Select(n => new [] { (object)n });
 
         [Theory]
         [MemberData(nameof(CharacterCounts))]