diff --git a/.github/external_ci/appveyor.yml b/.github/external_ci/appveyor.yml
index 126ed99b35..ee05af37a9 100644
--- a/.github/external_ci/appveyor.yml
+++ b/.github/external_ci/appveyor.yml
@@ -48,7 +48,7 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       configuration: Release
       platform: x86
-      CXX_FLAGS: "/W4 /WX"
+      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /W4 /WX"
       CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
       GENERATOR: Visual Studio 16 2019
 
diff --git a/include/nlohmann/detail/conversions/from_json.hpp b/include/nlohmann/detail/conversions/from_json.hpp
index 522292247f..42240ea28d 100644
--- a/include/nlohmann/detail/conversions/from_json.hpp
+++ b/include/nlohmann/detail/conversions/from_json.hpp
@@ -48,7 +48,7 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #ifdef JSON_HAS_CPP_17
-#ifndef JSON_USE_IMPLICIT_CONVERSIONS
+#if !JSON_USE_IMPLICIT_CONVERSIONS
 template<typename BasicJsonType, typename T>
 void from_json(const BasicJsonType& j, std::optional<T>& opt)
 {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 02441416bf..e45958b819 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -4721,7 +4721,7 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #ifdef JSON_HAS_CPP_17
-#ifndef JSON_USE_IMPLICIT_CONVERSIONS
+#if !JSON_USE_IMPLICIT_CONVERSIONS
 template<typename BasicJsonType, typename T>
 void from_json(const BasicJsonType& j, std::optional<T>& opt)
 {
diff --git a/tests/src/unit-conversions.cpp b/tests/src/unit-conversions.cpp
index 9adbc935af..a713c93aa1 100644
--- a/tests/src/unit-conversions.cpp
+++ b/tests/src/unit-conversions.cpp
@@ -1598,7 +1598,7 @@ TEST_CASE("JSON to enum mapping")
 
 
 #ifdef JSON_HAS_CPP_17
-#ifndef JSON_USE_IMPLICIT_CONVERSIONS
+#if !JSON_USE_IMPLICIT_CONVERSIONS
 TEST_CASE("std::optional")
 {
     SECTION("null")