VS 2008 : Randomize Dictionary Of Complex Types?

I wrote the following function to randomize a dictionary object:

Friend Function RandomizeDictionary(Of T)(ByVal oCollection As Dictionary(Of String, T)) As Dictionary(Of String, T)
Try
Dim oResult As Dictionary(Of String, T) = Nothing

[code]....

This code works fine on simple dictionary objects, but when I use it with complex types I get a type cast exception. For example, I have an object X that Inherits Dictionary (Of String, Class Y). If I call the randomize function on object X, I get the type cast exception. I don't really understand why it cares what the type of the dictionary value is, I thought that was the point of using generics.

In summary the following all work:

Dim x as New Dictionary (Of String, String)
Dim x as New Dictionary (Of String, Boolean)
Dim x as New Dictionary (Of String, Integer)

This does not:

Dim x as New Class X
Class X Inherits Dictionary (Of String, Class Y)