The reference may or may not be valid for the active platform

I have created a Pocket PC Class Library, lets call it myClass.dll

Created a PPC Windows Application.
Added a reference to myClass.dll using browse, get warning "The reference
'myClass.dll' may or may not be valid for the active platform. Add references
with care to ensure your application will run correctly"

Works OK until VS.NET is closed.
Load VS.NET back up and it says it can't find "myClass.dll" and it removes
the reference to it. I can add the reference again, browsing to the same
location as last time. All works OK again until VS.NET is closed and we start
all over.

Any ideas?
Chris.