FluentAssertions Type check

I try to use FluentAssertions to check in my UnitTest, that the type of a property in a list of items is of a certain type.

myObj.Items.OfType<TypeA>().Single()
            .MyProperty1.GetType()
                .Should().BeOfType<TypeB>();

Unfortunately, my test fails with the following error message:

Expected type to be TypeB, but found System.RuntimeType.

Why does it say, that it found System.RuntimeType? I used the debugger to verify, that MyProperty1is of type TypeB and it is... am I using .BeOfType<> wrong?

Thanks in advance