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

[Eric]

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
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