Inherit from a .cs object

I have a VB codebehind to an aspx file.. which I am telling to inherit from a .cs class in a dll that I have referenced in the class. Everything compiles just fine, I can reference the member variables to the base class in the code, but when I try to run the page, I get the following error...

System.MissingFieldException: Field not found: CustomPage.Web.UI.Page.SecurityString


It seems like its not really inheriting even though it compiles fine... can anyone tell me what I may be missing?

You said you referenced the cs file right? Could you supply an example of how you are calling the cs file?