diff --git a/lib/open_api_spex/cast/utils.ex b/lib/open_api_spex/cast/utils.ex
index 5be0b02a..c2ba810b 100644
--- a/lib/open_api_spex/cast/utils.ex
+++ b/lib/open_api_spex/cast/utils.ex
@@ -73,9 +73,19 @@ defmodule OpenApiSpex.Cast.Utils do
   - If multiple `content-type` headers are found, the function will only return the value of the first one.
 
   """
-  @spec content_type_from_header(Plug.Conn.t()) :: String.t() | nil
-  def content_type_from_header(conn = %Plug.Conn{}) do
-    case Plug.Conn.get_req_header(conn, "content-type") do
+  @spec content_type_from_header(Plug.Conn.t(), :request | :response) ::
+          String.t() | nil
+  def content_type_from_header(conn = %Plug.Conn{}, header_location \\ :request) do
+    content_type =
+      case header_location do
+        :request ->
+          Plug.Conn.get_req_header(conn, "content-type")
+
+        :response ->
+          Plug.Conn.get_resp_header(conn, "content-type")
+      end
+
+    case content_type do
       [header_value | _] ->
         header_value
         |> String.split(";")
diff --git a/lib/open_api_spex/test/test_assertions.ex b/lib/open_api_spex/test/test_assertions.ex
index 42f95b2e..6d089024 100644
--- a/lib/open_api_spex/test/test_assertions.ex
+++ b/lib/open_api_spex/test/test_assertions.ex
@@ -136,7 +136,7 @@ defmodule OpenApiSpex.TestAssertions do
     case operation_lookup[operation_id] do
       nil ->
         flunk(
-          "Failed to resolve schema. Unable to find a response for operation_id: #{operation_id} for response status code: #{conn.status}"
+          "Failed to resolve a response schema for operation_id: #{operation_id} for status code: #{conn.status}"
         )
 
       operation ->
@@ -154,20 +154,19 @@ defmodule OpenApiSpex.TestAssertions do
           ) ::
             term | no_return
     defp validate_operation_response(conn, %Operation{operationId: operation_id} = operation, spec) do
-      content_type = Utils.content_type_from_header(conn)
+      content_type = Utils.content_type_from_header(conn, :response)
 
       resolved_schema =
-        get_in(operation, [
-          Access.key!(:responses),
-          Access.key!(conn.status),
-          Access.key!(:content),
-          content_type,
-          Access.key!(:schema)
-        ])
+        operation
+        |> Map.get(:responses, %{})
+        |> Map.get(conn.status, %{})
+        |> Map.get(:content, %{})
+        |> Map.get(content_type, %{})
+        |> Map.get(:schema)
 
       if is_nil(resolved_schema) do
         flunk(
-          "Failed to resolve schema! Unable to find a response for operation_id: #{operation_id} for response status code: #{conn.status} and content type #{content_type}"
+          "Failed to resolve a response schema for operation_id: #{operation_id} for status code: #{conn.status} and content type: #{content_type}"
         )
       end
 
diff --git a/test/test_assertions_test.exs b/test/test_assertions_test.exs
index 0959eef5..9174b5e2 100644
--- a/test/test_assertions_test.exs
+++ b/test/test_assertions_test.exs
@@ -129,14 +129,11 @@ defmodule OpenApiSpex.TestAssertionsTest do
       conn = OpenApiSpexTest.Router.call(conn, [])
       assert conn.status == 200
 
-      try do
-        TestAssertions.assert_operation_response(conn, "not_a_real_operation_id")
-        raise RuntimeError, "Should flunk"
-      rescue
-        e in ExUnit.AssertionError ->
-          assert e.message =~
-                   "Failed to resolve schema. Unable to find a response for operation_id: not_a_real_operation_id for response status code: 200"
-      end
+      assert_raise(
+        ExUnit.AssertionError,
+        ~r/Failed to resolve a response schema for operation_id: not_a_real_operation_id for status code: 200/,
+        fn -> TestAssertions.assert_operation_response(conn, "not_a_real_operation_id") end
+      )
     end
 
     test "invalid schema" do
@@ -149,14 +146,29 @@ defmodule OpenApiSpex.TestAssertionsTest do
 
       assert conn.status == 200
 
-      try do
-        TestAssertions.assert_operation_response(conn, "showPetById")
-        raise RuntimeError, "Should flunk"
-      rescue
-        e in ExUnit.AssertionError ->
-          assert e.message =~
-                   "Value does not conform to schema PetResponse: Failed to cast value to one of: no schemas validate at"
-      end
+      assert_raise(
+        ExUnit.AssertionError,
+        ~r/Value does not conform to schema PetResponse: Failed to cast value to one of: no schemas validate at/,
+        fn -> TestAssertions.assert_operation_response(conn, "showPetById") end
+      )
+    end
+
+    test "returns an error when the response content-type does not match the schema" do
+      conn =
+        :get
+        |> Plug.Test.conn("/api/pets")
+        |> Plug.Conn.put_req_header("content-type", "application/json")
+        |> Plug.Conn.put_resp_header("content-type", "unexpected-content-type")
+
+      conn = OpenApiSpexTest.Router.call(conn, [])
+
+      assert conn.status == 200
+
+      assert_raise(
+        ExUnit.AssertionError,
+        ~r/Failed to resolve a response schema for operation_id: showPetById for status code: 200 and content type: unexpected-content-type/,
+        fn -> TestAssertions.assert_operation_response(conn, "showPetById") end
+      )
     end
   end
 end