Trying to debug custom FxCop rule fails because FxCopCmd can't load the assembly (CA0055)

I'm trying to debug a custom FxCop rule I wrote by running: devenv /debugexe FxCopCmd.exe ...

Initially I got some errors about missing references, but I managed to fix those by using the following command line:

    FxCopCmd.exe /searchgac /d:"SomeDir" /file:"MyAssembly.dll" /rule:"MyCustomRules.dll" /console

Now, I'm getting the following error:

    Project : error : CA0055 : Could not load MyAssembly.dll. Signature is not valid.
    Project : error : CA0052 : No targets were selected.

I made sure that MyAssembly.dll is signed with my test signing certificate, and that all of the certificates in the certification path are in the "Trusted Root Certification Authorities" (of the current user) but it doesn't help.
Does anyone know what this error means?

Thanks!