diff --git a/config/config.exs b/config/config.exs
index f835efa..5709875 100644
--- a/config/config.exs
+++ b/config/config.exs
@@ -2,9 +2,10 @@
 # and its dependencies with the aid of the Mix.Config module.
 use Mix.Config
 
-  config :sparkpost, api_endpoint: "https://api.sparkpost.com/api/v1/"
-  config :sparkpost, api_key: "YOUR API KEY HERE"
-  config :sparkpost, http_timeout: 5000
+config :sparkpost,
+  api_endpoint: "https://api.sparkpost.com/api/v1/",
+  api_key: "YOUR API KEY HERE",
+  http_timeout: 5000
 
 # This configuration is loaded before any dependency and is restricted
 # to this project. If another project depends on this project, this
diff --git a/lib/endpoint.ex b/lib/endpoint.ex
index 9ca19a4..0528c9c 100644
--- a/lib/endpoint.ex
+++ b/lib/endpoint.ex
@@ -10,11 +10,19 @@ defmodule SparkPost.Endpoint do
   Make a request to the SparkPost API.
 
   ## Parameters
-    - method: HTTP request method as atom (:get, :post, ...)
-    - endpoint: SparkPost API endpoint as string ("transmissions", "templates", ...)
-    - options: keyword of optional elements including:
-      - :params: keyword of query parameters
-      - :body: request body (string)
+    - `method`: HTTP 1.1 request method as an atom:
+      - `:delete`
+      - `:get`
+      - `:head`
+      - `:options`
+      - `:patch` 
+      - `:post`
+      - `:put`
+    - `endpoint`: SparkPost API endpoint as string ("transmissions", "templates", ...)
+    - `body`: A Map that will be encoded to JSON to be sent as the body of the request (defaults to empty)
+    - `headers`: A Map of headers of the form %{"Header-Name" => "Value"} to be sent with the request
+    - `options`: A Keyword list of optional elements including:
+      - `:params`: A Keyword list of query parameters
 
   ## Example
     List transmissions for the "ElixirRox" campaign:
@@ -24,34 +32,24 @@ defmodule SparkPost.Endpoint do
           "id" => "102258558346809186", "name" => "102258558346809186",
           "state" => "Success"}, ...], status_code: 200}
   """
-  def request(method, endpoint, options) do
-    url = if Keyword.has_key?(options, :params) do
-      Application.get_env(:sparkpost, :api_endpoint, @default_endpoint) <> endpoint
-        <> "?" <> URI.encode_query(options[:params])
-    else
-      Application.get_env(:sparkpost, :api_endpoint, @default_endpoint) <> endpoint
-    end
-
-    reqopts = if method in [:get, :delete] do
-      [ headers: base_request_headers() ]
-    else
-      [
-        headers: ["Content-Type": "application/json"] ++ base_request_headers(),
-        body: encode_request_body(options[:body])
-      ]
-    end
+  def request(method, endpoint, body \\ %{},  headers \\ %{}, options \\ []) do
+    url = Application.get_env(:sparkpost, :api_endpoint, @default_endpoint) <> endpoint
 
-    reqopts = [timeout: Application.get_env(:sparkpost, :http_timeout, 5000)] ++ reqopts
+    {:ok, request_body} = encode_request_body(body)
+    
+    request_headers = if method in [:get, :delete] do
+        headers 
+      else
+        Map.merge(headers, %{"Content-Type": "application/json"})
+      end
+      |> Map.merge(base_request_headers)
 
-    %{status_code: status_code, body: json} = HTTPotion.request(method, url, reqopts)
+    timeout = Application.get_env(:sparkpost, :http_timeout, 5000)
 
-    body = decode_response_body(json)
+    request_options = Keyword.put(options, :timeout, timeout)
 
-    if Map.has_key?(body, :errors) do
-      %SparkPost.Endpoint.Error{ status_code: status_code, errors: body.errors }
-    else
-      %SparkPost.Endpoint.Response{ status_code: status_code, results: body.results }
-    end
+    HTTPoison.request(method, url, request_body, request_headers, request_options)
+    |> handle_response
   end
 
   def marshal_response(response, struct_type, subkey\\nil)
@@ -72,16 +70,30 @@ defmodule SparkPost.Endpoint do
     response
   end
 
+  defp handle_response({:ok, %HTTPoison.Response{status_code: code, body: body}}) when code >= 200 and code < 300 do
+    decoded_body = decode_response_body(body)
+    %SparkPost.Endpoint.Response{status_code: 200, results: decoded_body.results}
+  end
+
+  defp handle_response({:ok, %HTTPoison.Response{status_code: code, body: body}}) when code >= 400 do
+    decoded_body = decode_response_body(body)
+    if Map.has_key?(decoded_body, :errors) do
+      %SparkPost.Endpoint.Error{status_code: code, errors: decoded_body.errors}
+    end
+  end
+
   defp base_request_headers() do
     {:ok, version} = :application.get_key(:sparkpost, :vsn)
-    [
+    %{
       "User-Agent": "elixir-sparkpost/" <> to_string(version),
       "Authorization": Application.get_env(:sparkpost, :api_key)
-    ]
+    }
   end
 
+  # Do not try to remove nils from an empty map 
+  defp encode_request_body(body) when is_map(body) and map_size(body) == 0, do: {:ok, ""}
   defp encode_request_body(body) do
-    body |> Washup.filter |> Poison.encode!
+    body |> Washup.filter |> Poison.encode
   end
 
   defp decode_response_body(body) do
diff --git a/lib/mockserver.ex b/lib/mockserver.ex
index 044bb02..20ef952 100644
--- a/lib/mockserver.ex
+++ b/lib/mockserver.ex
@@ -34,6 +34,6 @@ defmodule SparkPost.MockServer do
   end
 
   def mk_http_resp(status_code, body) do
-    fn (_method, _url, _opts) -> %{status_code: status_code, body: body} end
+    fn (_method, _url, _body, _headers, _opts) -> {:ok, %HTTPoison.Response{status_code: status_code, body: body}} end
   end
 end
diff --git a/lib/transmission.ex b/lib/transmission.ex
index be56813..d06b580 100644
--- a/lib/transmission.ex
+++ b/lib/transmission.ex
@@ -126,7 +126,7 @@ defmodule SparkPost.Transmission do
       recipients: Recipient.to_recipient_list(body.recipients),
       content: Content.to_content(body.content)
     }
-    response = Endpoint.request(:post, "transmissions", [body: body])
+    response = Endpoint.request(:post, "transmissions", body)
     Endpoint.marshal_response(response, Transmission.Response)
   end
 
@@ -150,7 +150,7 @@ defmodule SparkPost.Transmission do
              substitution_data: ""}
   """
   def get(transid) do
-    response = Endpoint.request(:get, "transmissions/" <> transid, [])
+    response = Endpoint.request(:get, "transmissions/" <> transid)
     Endpoint.marshal_response(response, __MODULE__, :transmission)
   end
 
@@ -179,7 +179,7 @@ defmodule SparkPost.Transmission do
         return_path: :required, state: "Success", substitution_data: nil}]
   """
   def list(filters\\[]) do
-    response = Endpoint.request(:get, "transmissions", [params: filters])
+    response = Endpoint.request(:get, "transmissions", %{}, %{}, [params: filters])
     case response do
       %Endpoint.Response{} ->
         Enum.map(response.results, fn (trans) -> struct(__MODULE__, trans) end)
diff --git a/lib/washup.ex b/lib/washup.ex
index 668a0dd..8606e5c 100644
--- a/lib/washup.ex
+++ b/lib/washup.ex
@@ -10,6 +10,8 @@ defmodule Washup do
       iex> jenny = %{name: "Jennifer", age: 27, rank: "Captain", pets: nil}
       iex> Washup.filter(jenny)
       %{name: "Jennifer", age: 27, rank: "Captain"}
+      iex> Washup.filter("Plain String")
+      "Plain String" 
   """
   def filter(it) do
     cond do
diff --git a/mix.exs b/mix.exs
index 4b434d4..b55a6e9 100644
--- a/mix.exs
+++ b/mix.exs
@@ -20,13 +20,12 @@ defmodule SparkPost.Mixfile do
   end
 
   def application do
-    [applications: [:httpotion]]
+    [applications: [:httpoison]]
   end
 
   defp deps do
     [
-      {:ibrowse, github: "cmullaparthi/ibrowse", tag: "v4.1.2"},
-      {:httpotion, "~> 2.1.0"},
+      {:httpoison, "~> 0.9"},
       {:poison, "~> 1.5"},
       {:mock, "~> 0.1.1", only: :test},
       {:excoveralls, "~> 0.4", only: :test},
diff --git a/mix.lock b/mix.lock
index b3ac27f..22d7615 100644
--- a/mix.lock
+++ b/mix.lock
@@ -1,17 +1,20 @@
 %{"bunt": {:hex, :bunt, "0.1.6", "5d95a6882f73f3b9969fdfd1953798046664e6f77ec4e486e6fafc7caad97c6f", [:mix], []},
-  "certifi": {:hex, :certifi, "0.3.0", "389d4b126a47895fe96d65fcf8681f4d09eca1153dc2243ed6babad0aac1e763", [:rebar3], []},
+  "certifi": {:hex, :certifi, "0.4.0", "a7966efb868b179023618d29a407548f70c52466bf1849b9e8ebd0e34b7ea11f", [:rebar3], []},
   "credo": {:hex, :credo, "0.4.3", "29fe87aa2ef3c19bf8dd909594b9b79d9e2ed2857a153c00eb3f697f41cb6782", [:mix], [{:bunt, "~> 0.1.6", [hex: :bunt, optional: false]}]},
   "earmark": {:hex, :earmark, "0.2.1", "ba6d26ceb16106d069b289df66751734802777a3cbb6787026dd800ffeb850f3", [:mix], []},
   "ex_doc": {:hex, :ex_doc, "0.11.3", "bb16cb3f4135d880ce25279dc19a9d70802bc4f4942f0c3de9e4862517ae3ace", [:mix], [{:earmark, "~> 0.1.17 or ~> 0.2", [hex: :earmark, optional: true]}]},
-  "excoveralls": {:hex, :excoveralls, "0.4.5", "1508e1c7f373f82805975c633e2468a83898b2b902acf79e7359486d71186ea3", [:mix], [{:hackney, ">= 0.12.0", [hex: :hackney, optional: false]}, {:exjsx, "~> 3.0", [hex: :exjsx, optional: false]}]},
+  "excoveralls": {:hex, :excoveralls, "0.4.5", "1508e1c7f373f82805975c633e2468a83898b2b902acf79e7359486d71186ea3", [:mix], [{:exjsx, "~> 3.0", [hex: :exjsx, optional: false]}, {:hackney, ">= 0.12.0", [hex: :hackney, optional: false]}]},
   "exjsx": {:hex, :exjsx, "3.2.0", "7136cc739ace295fc74c378f33699e5145bead4fdc1b4799822d0287489136fb", [:mix], [{:jsx, "~> 2.6.2", [hex: :jsx, optional: false]}]},
-  "hackney": {:hex, :hackney, "1.4.8", "c8c6977ed55cc5095e3929f6d94a6f732dd2e31ae42a7b9236d5574ec3f5be13", [:rebar3], [{:ssl_verify_hostname, "1.0.5", [hex: :ssl_verify_hostname, optional: false]}, {:mimerl, "1.0.2", [hex: :mimerl, optional: false]}, {:idna, "1.0.3", [hex: :idna, optional: false]}, {:certifi, "0.3.0", [hex: :certifi, optional: false]}]},
+  "hackney": {:hex, :hackney, "1.6.1", "ddd22d42db2b50e6a155439c8811b8f6df61a4395de10509714ad2751c6da817", [:rebar3], [{:certifi, "0.4.0", [hex: :certifi, optional: false]}, {:idna, "1.2.0", [hex: :idna, optional: false]}, {:metrics, "1.0.1", [hex: :metrics, optional: false]}, {:mimerl, "1.0.2", [hex: :mimerl, optional: false]}, {:ssl_verify_fun, "1.1.0", [hex: :ssl_verify_fun, optional: false]}]},
+  "httpoison": {:hex, :httpoison, "0.9.0", "68187a2daddfabbe7ca8f7d75ef227f89f0e1507f7eecb67e4536b3c516faddb", [:mix], [{:hackney, "~> 1.6.0", [hex: :hackney, optional: false]}]},
   "httpotion": {:hex, :httpotion, "2.1.0", "3fe84fbd13d4560c2514da656d022b1191a079178ee4992d245fc3c33c01ee18", [:mix], []},
   "ibrowse": {:git, "https://github.com/cmullaparthi/ibrowse.git", "ea3305d21f37eced4fac290f64b068e56df7de80", [tag: "v4.1.2"]},
-  "idna": {:hex, :idna, "1.0.3", "d456a8761cad91c97e9788c27002eb3b773adaf5c893275fc35ba4e3434bbd9b", [:rebar3], []},
+  "idna": {:hex, :idna, "1.2.0", "ac62ee99da068f43c50dc69acf700e03a62a348360126260e87f2b54eced86b2", [:rebar3], []},
   "jsx": {:hex, :jsx, "2.6.2", "213721e058da0587a4bce3cc8a00ff6684ced229c8f9223245c6ff2c88fbaa5a", [:mix, :rebar], []},
   "meck": {:hex, :meck, "0.8.4", "59ca1cd971372aa223138efcf9b29475bde299e1953046a0c727184790ab1520", [:rebar, :make], []},
+  "metrics": {:hex, :metrics, "1.0.1", "25f094dea2cda98213cecc3aeff09e940299d950904393b2a29d191c346a8486", [:rebar3], []},
   "mimerl": {:hex, :mimerl, "1.0.2", "993f9b0e084083405ed8252b99460c4f0563e41729ab42d9074fd5e52439be88", [:rebar3], []},
   "mock": {:hex, :mock, "0.1.1", "e21469ca27ba32aa7b18b61699db26f7a778171b21c0e5deb6f1218a53278574", [:mix], [{:meck, "~> 0.8.2", [hex: :meck, optional: false]}]},
   "poison": {:hex, :poison, "1.5.2", "560bdfb7449e3ddd23a096929fb9fc2122f709bcc758b2d5d5a5c7d0ea848910", [:mix], []},
+  "ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.0", "edee20847c42e379bf91261db474ffbe373f8acb56e9079acb6038d4e0bf414f", [:rebar, :make], []},
   "ssl_verify_hostname": {:hex, :ssl_verify_hostname, "1.0.5", "2e73e068cd6393526f9fa6d399353d7c9477d6886ba005f323b592d389fb47be", [:make], []}}
diff --git a/test/endpoint_test.exs b/test/endpoint_test.exs
index ad638ef..291dbdf 100644
--- a/test/endpoint_test.exs
+++ b/test/endpoint_test.exs
@@ -26,39 +26,26 @@ defmodule SparkPost.EndpointTest do
     end
   end
 
-  test "Endpoint.request forms correct URLs" do
-    base_url = Application.get_env(:sparkpost, :api_endpoint)
-    endpt = "transmissions"
-    params = [campaign_id: "campaign101"]
-    paramstr = URI.encode_query(params)
-    respfn = MockServer.mk_resp
-    with_mock HTTPotion, [request: fn(method, url, opts) ->
-      assert url == base_url <> endpt <> "?" <> paramstr
-      respfn.(method, url, opts)
-    end] do
-      Endpoint.request(:get, "transmissions", [params: params])
-    end
-  end
-
   test "Endpoint.request succeeds with Endpoint.Response" do
-    with_mock HTTPotion, [request: MockServer.mk_resp] do
-      Endpoint.request(:get, "transmissions", [])
+    with_mock HTTPoison, [request: fn(_, _, _, _, _) -> 
+     r = MockServer.mk_resp
+     r.(nil, nil, nil, nil, nil)
+    end] do
+      Endpoint.request(:get, "transmissions", %{})
     end
   end
 
   test "Endpoint.request populates Endpoint.Response"  do
     status_code = 200
     results = Poison.decode!(MockServer.create_json, [keys: :atoms]).results
-    with_mock HTTPotion, [request: MockServer.mk_resp] do
-      resp = %Endpoint.Response{} = Endpoint.request(
-        :get, "transmissions", [])
-
+    with_mock HTTPoison, [request: MockServer.mk_resp] do
+      resp = %Endpoint.Response{} = Endpoint.request(:get, "transmissions", %{}, %{}, [])
       assert %Endpoint.Response{status_code: ^status_code, results: ^results} = resp
     end
   end
 
   test "Endpoint.request fails with Endpoint.Error" do
-    with_mock HTTPotion, [request: MockServer.mk_fail] do
+    with_mock HTTPoison, [request: MockServer.mk_fail] do
       %Endpoint.Error{} = Endpoint.request(
         :get, "transmissions", [])
     end
@@ -67,7 +54,7 @@ defmodule SparkPost.EndpointTest do
   test "Endpoint.request populates Endpoint.Error" do
     status_code = 400
     errors = Poison.decode!(MockServer.create_fail_json, [keys: :atoms]).errors
-    with_mock HTTPotion, [request: MockServer.mk_fail] do
+    with_mock HTTPoison, [request: MockServer.mk_fail] do
       resp = %Endpoint.Error{} = Endpoint.request(
         :get, "transmissions", [])
 
@@ -77,42 +64,42 @@ defmodule SparkPost.EndpointTest do
 
   test "Endpoint.request includes the core HTTP headers" do
     respfn = MockServer.mk_resp
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
       Enum.each(Headers.for_method(method), fn {header, tester} ->
         header_atom = String.to_atom(header)
-        assert Keyword.has_key?(opts[:headers], header_atom), "#{header} header required for #{method} requests"
-        assert tester.(opts[:headers][header_atom]), "Malformed header: #{header}.  See Headers module in #{__ENV__.file} for formatting rules."
+        assert Map.has_key?(headers, header_atom), "#{header} header required for #{method} requests"
+        assert tester.(headers[header_atom]), "Malformed header: #{header}.  See Headers module in #{__ENV__.file} for formatting rules."
       end)
-      respfn.(method, url, opts)
+      respfn.(method, url, body, headers, opts)
     end
     ] do
       Enum.each([:get, :post, :put, :delete], fn method ->
-        Endpoint.request(method, "transmissions", []) end)
+        Endpoint.request(method, "transmissions", %{}, %{}, []) end)
     end
   end
 
   test "Endpoint.request includes request bodies for appropriate methods" do
     respfn = MockServer.mk_resp
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
-      assert opts[:body] == "{}"
-      respfn.(method, url, opts)
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
+      assert body == ""
+      respfn.(method, url, body, headers, opts)
     end
     ] do
-      Endpoint.request(:post, "transmissions", [body: %{}])
-      Endpoint.request(:put, "transmissions", [body: %{}])
+      Endpoint.request(:post, "transmissions", %{}, %{}, [])
+      Endpoint.request(:put, "transmissions", %{}, %{}, [])
     end
   end
 
   test "Endpoint.request includes request timeout" do
     respfn = MockServer.mk_resp
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
       assert Keyword.has_key?(opts, :timeout)
-      respfn.(method, url, opts)
+      respfn.(method, url, body, headers, opts)
     end
     ] do
-      Endpoint.request(:post, "transmissions", [])
-      Endpoint.request(:put, "transmissions", [])
-      Endpoint.request(:get, "transmissions", [])
+      Endpoint.request(:post, "transmissions", %{}, %{}, [])
+      Endpoint.request(:put, "transmissions", %{}, %{}, [])
+      Endpoint.request(:get, "transmissions", %{}, %{}, [])
     end
   end
 end
diff --git a/test/sparkpost_test.exs b/test/sparkpost_test.exs
index b9e3265..91a5d00 100644
--- a/test/sparkpost_test.exs
+++ b/test/sparkpost_test.exs
@@ -6,7 +6,7 @@ defmodule SparkPostTest do
   import Mock
 
   test "send succeeds with a Transmission.Response" do
-    with_mock HTTPotion, [request: MockServer.mk_resp] do
+    with_mock HTTPoison, [request: MockServer.mk_resp] do
       resp = SparkPost.send(
         to: "you@there.com",
         from: "me@here.com",
@@ -19,7 +19,7 @@ defmodule SparkPostTest do
   end
 
   test "send fails with a Endpoint.Error" do
-    with_mock HTTPotion, [request: MockServer.mk_fail] do
+    with_mock HTTPoison, [request: MockServer.mk_fail] do
       resp = SparkPost.send(
         to: "you@there.com",
         from: "me@here.com",
@@ -37,8 +37,8 @@ defmodule SparkPostTest do
     subject = "Elixir and SparkPost..."
     text = "Raw text email is boring"
     html = "<marquee>Rich text email is terrifying</marquee>"
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
-      inreq = Poison.decode!(opts[:body], [keys: :atoms])
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
+      inreq = Poison.decode!(body, [keys: :atoms])
       assert Recipient.to_recipient_list(inreq.recipients) == Recipient.to_recipient_list(to)
       assert Content.to_content(inreq.content) == %Content.Inline{
         from: Address.to_address(from),
@@ -46,7 +46,7 @@ defmodule SparkPostTest do
         text: text,
         html: html
       }
-      MockServer.mk_resp.(method, url, opts)
+      MockServer.mk_resp.(method, url, body, headers, opts)
     end] do
       SparkPost.send(
         to: to,
diff --git a/test/transmission_test.exs b/test/transmission_test.exs
index fda2f9b..baace42 100644
--- a/test/transmission_test.exs
+++ b/test/transmission_test.exs
@@ -50,7 +50,7 @@ defmodule SparkPost.TransmissionTest do
 
   defmodule TestRequests do
     def test_send(req, test_fn) do
-      with_mock HTTPotion, [
+      with_mock HTTPoison, [
         request: handle_send(test_fn)
       ] do
         Transmission.send(req)
@@ -58,8 +58,8 @@ defmodule SparkPost.TransmissionTest do
     end
 
     defp handle_send(response_test_fn) do
-      fn (method, url, opts) ->
-        req = Poison.decode!(opts[:body], [keys: :atoms])
+      fn (method, url, body, headers, opts) ->
+        req = Poison.decode!(body, [keys: :atoms])
         fullreq = struct(Transmission, %{
           req |
           options: struct(Transmission.Options, req.options),
@@ -67,12 +67,12 @@ defmodule SparkPost.TransmissionTest do
           content: Content.to_content(req.content)
         })
         response_test_fn.(fullreq)
-        MockServer.mk_resp.(method, url, opts)
+        MockServer.mk_resp.(method, url, body, headers, opts)
       end
     end
 
     defp parse_recipients_field(lst) when is_list(lst) do
-      Enum.map(lst, fn recip -> 
+      Enum.map(lst, fn recip ->
         struct(Recipient, parse_recipient(recip))
       end)
     end
@@ -95,14 +95,14 @@ defmodule SparkPost.TransmissionTest do
   end
 
   test "Transmission.send succeeds with Transmission.Response" do
-    with_mock HTTPotion, [request: MockServer.mk_resp] do
+    with_mock HTTPoison, [request: MockServer.mk_resp] do
       resp = Transmission.send(TestStructs.basic_transmission)
       assert %Transmission.Response{} = resp
     end
   end
 
   test "Transmission.send fails with Endpoint.Error" do
-    with_mock HTTPotion, [request: MockServer.mk_fail] do
+    with_mock HTTPoison, [request: MockServer.mk_fail] do
       req = TestStructs.basic_transmission
       resp = Transmission.send(req)
       assert %SparkPost.Endpoint.Error{} = resp
@@ -110,9 +110,9 @@ defmodule SparkPost.TransmissionTest do
   end
 
   test "Transmission.send emits a POST" do
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
       assert method == :post
-      MockServer.mk_resp.(method, url, opts)
+      MockServer.mk_resp.(method, url, body, headers, opts)
     end] do
       Transmission.send(TestStructs.basic_transmission)
     end
@@ -240,7 +240,7 @@ defmodule SparkPost.TransmissionTest do
   end
 
   test "Transmission.list succeeds with a list of Transmission" do
-    with_mock HTTPotion, [request: MockServer.mk_list] do
+    with_mock HTTPoison, [request: MockServer.mk_list] do
       resp = Transmission.list
       assert is_list(resp)
       Enum.each(resp, fn r -> assert %Transmission{} = r end)
@@ -248,16 +248,16 @@ defmodule SparkPost.TransmissionTest do
   end
 
   test "Transmission.list fails with Endpoint.Error" do
-    with_mock HTTPotion, [request: MockServer.mk_fail] do
+    with_mock HTTPoison, [request: MockServer.mk_fail] do
       resp = Transmission.list
       assert %SparkPost.Endpoint.Error{} = resp
     end
   end
 
   test "Transmission.list emits a GET" do
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
       assert method == :get
-      MockServer.mk_list.(method, url, opts)
+      MockServer.mk_list.(method, url, body, headers, opts)
     end] do
       Transmission.list
     end
@@ -267,9 +267,9 @@ defmodule SparkPost.TransmissionTest do
   end
 
   test "Transmission.get emits a GET" do
-    with_mock HTTPotion, [request: fn (method, url, opts) ->
+    with_mock HTTPoison, [request: fn (method, url, body, headers, opts) ->
       assert method == :get
-      MockServer.mk_get.(method, url, opts)
+      MockServer.mk_get.(method, url, body, headers, opts)
     end] do
       Transmission.get("TRANSMISSION_ID")
     end