[Haskell-cafe] "Could not list sources" warning in sandboxed project with add-source deps

I'm getting a warning

Warning: Could not list sources of the add-source dependency 'xyz'.> Skipping the timestamp check. when building a sandboxed project with add-source dependencies.  I tried increasing the verbosity of cabal, but it still won't tell me what's going wrong so I can fix it. Any suggestions on how to debug or fix this? thanks,--Eric