"Error reading key file" when compiling VS.NET web app

[CRogers]

I have a web app whose assembly is strongly named. Compilation within VS.NET works fine. Compiling during a build fails with the error

vbc : error BC30140: Error creating assembly manifest: Error reading key file 'path\to\my.snk' -- The system cannot find the path specified.

Compiling non-web apps using this same key file works fine as well.

Any advice?