testfx: IsInstanceOfType fails on COM-objects

I was using the Microsoft.VisualStudio.QualityTools.UnitTestFramework in Visual Studio 2015 so far.

Here is a TestMethod that worked until now:

<TestMethod> Public Sub GetStringParameterTest()
     Dim rootParamSet As ParameterSet = XXXUtilities.GetOrCreateRootParameterSet(_workDocument, ROOT_PARAM_SET_NAME)
     Dim paramSubSet As ParameterSet = XXXUtilities.GetOrCreateParameterSubSet(rootParamSet, PARAM_SUBSET_NAME, False)
     Dim stringParameter As StrParam = XXXUtilities.GetOrCreateStringParameter(paramSubSet, "UNITTEST_STRING_PARAM", False)

     Dim actual As StrParam = XXXUtilities.GetStringParameter(paramSubSet, "UNITTEST_STRING_PARAM")

     Assert.IsNotNull(actual)
     Assert.IsInstanceOfType(actual, GetType(StrParam))
     Dim name As String = XXXUtilities.GetParameterShortName(actual.Name)
     Assert.AreEqual("UNITTEST_STRING_PARAM", name)
 End Sub

Expected behaviour: Assert.IsInstanceOfType(actual, GetType(StrParam)) returns true. StrParam is a Type defined in a COM-DLL.

Today, I tried to use the Microsoft.VisualStudio.TestPlatform.TestFramework instead, because I need to use data-driven TestMethods with DataRows.

Actual behavior: the same test now fails with this error:

Result Message: Assert.IsInstanceOfType failed. Expected type:<KnowledgewareTypeLib.StrParam>. Actual type:<System.__ComObject>.

Obviously, the type StrParam was lost in favor of the completely generic System.ComObject. This feels like a regression compared to the “old” behaviour.

What am I missing here?