Unable to cast object of type 'ASP.master' to type 'ASP.master'

[BillE]

I get an intermittent error when I start a debug session for a vb.net web
site which uses master/content pages:

"Unable to cast object of type 'ASP.master' to type 'ASP.master'"

I think this is not a bug in my code, because if I close Visual Studio and
open it again the problem is gone, for a while. It recurs after several
sessions.

Is there a remedy for this?

I'm using VS 2005.

Thanks
Bill