diff --git a/Octokit.Tests.Conventions/Exception/ReturnValueMismatchException.cs b/Octokit.Tests.Conventions/Exception/ReturnValueMismatchException.cs
new file mode 100644
index 00000000..121751d7
--- /dev/null
+++ b/Octokit.Tests.Conventions/Exception/ReturnValueMismatchException.cs
@@ -0,0 +1,23 @@
+using System;
+using System.Reflection;
+using System.Runtime.Serialization;
+
+namespace Octokit.Tests.Conventions
+{
+    public class ReturnValueMismatchException : Exception
+    {
+        public ReturnValueMismatchException(MethodInfo method, Type expected, Type actual)
+            : base(CreateMessage(method, expected, actual)) { }
+
+        public ReturnValueMismatchException(MethodInfo method, Type expected, Type actual, Exception innerException)
+            : base(CreateMessage(method, expected, actual), innerException) { }
+
+        protected ReturnValueMismatchException(SerializationInfo info, StreamingContext context)
+            : base(info, context) { }
+
+        static string CreateMessage(MethodInfo method, Type expected, Type actual)
+        {
+            return String.Format("Return value for {0}.{1} must be {2} but is {3}", method.DeclaringType.Name,  method.Name, expected, actual);
+        }
+    }
+}
\ No newline at end of file
diff --git a/Octokit.Tests.Conventions/Octokit.Tests.Conventions.csproj b/Octokit.Tests.Conventions/Octokit.Tests.Conventions.csproj
index 8979df85..8caa28f7 100644
--- a/Octokit.Tests.Conventions/Octokit.Tests.Conventions.csproj
+++ b/Octokit.Tests.Conventions/Octokit.Tests.Conventions.csproj
@@ -53,6 +53,7 @@
     <Compile Include="Exception\InterfaceHasAdditionalMethodsException.cs" />
     <Compile Include="Exception\InterfaceMissingMethodsException.cs" />
     <Compile Include="Exception\InterfaceNotFoundException.cs" />
+    <Compile Include="Exception\ReturnValueMismatchException.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="SyncObservableClients.cs" />
     <Compile Include="TypeExtensions.cs" />
diff --git a/Octokit.Tests.Conventions/SyncObservableClients.cs b/Octokit.Tests.Conventions/SyncObservableClients.cs
index c78e5a47..c78eb4cd 100644
--- a/Octokit.Tests.Conventions/SyncObservableClients.cs
+++ b/Octokit.Tests.Conventions/SyncObservableClients.cs
@@ -59,7 +59,11 @@ namespace Octokit.Tests.Conventions
             var mainReturnType = mainMethod.ReturnType;
             var observableReturnType = observableMethod.ReturnType;
             var expectedType = GetObservableExpectedType(mainReturnType);
-            Assert.Equal(expectedType, observableReturnType);
+
+            if (expectedType != observableReturnType)
+            {
+                throw new ReturnValueMismatchException(observableMethod, expectedType, observableReturnType);
+            }
         }
 
         private static Type GetObservableExpectedType(Type mainType)