How do you verify a non-public generic method is called?

[TestMethod]
public void Test_CallPrivateGeneric()
{
	var fakeCalculatorGeneric = Isolate.Fake.Instance<CalculatorGeneric<String>>();
	Isolate.Invoke.Method(fakeCalculatorGeneric, "DoSomethingPrivate");

	Isolate.Verify.NonPublic.WasCalled(fakeCalculatorGeneric, "DoSomethingPrivate");
}

public class CalculatorGeneric<T>
{
	public string DoSomething<T>(bool callMethod)
	{
		if (callMethod)
		{
			return DoSomethingPrivate<T>();
		}
		else
		{
			return "DidNotCalltheMethod";
		}
	}

	private string DoSomethingPrivate<T>()
	{
		return "foo";
	}
}

But when I ran this time, I saw this error: