Signing in Visual Studio 2005 - error using existing .pfx

Question

1. Built my project.

2. Created my .pfx using sn.exe -k (to create ShellTradingCCMPROD.pfx).

3. Copied the file to the application folder.

4. In VS 2005, I go to the Signing tab, 'Select From File' and browse to the .pfx file.

5. I hit 'Open' and I get the following error:

The file 'C:\2009.02.1.1\ShellTrading.CCM.WinUI\ShellTradingCCMPROD.pfx' could not be imported: Cannot find the requested object.

Other .pfx files work - any clues?