CType using a variable to hold the type doesn't work

[KyferEz]

So I do not know at design time if the var ms is going to be a stringreader or a memorystream. So I am trying to do basically this:

Dim ms as object = myfunction...
Dim mytype As Type = ms.GetType
Dim returnedobject = ser.Deserialize(CType(ms, mytype))

But I get an error saying "type mytype is not defined". How can I do this?