Cannot find references to other Pages in my project

Hello there,
I have two pages, say pageA and pageB, with code behind classes CodePageA and CodePageB. I would like to reference an instance of CodePageA in CodePageB, but I'm getting a 
Compiler Error Message: CS0246
Both of my classes are in the default namespace. Can anyone give a hint of whats going on? 


Thanks in advance.


...