ghc panic ((!!): index too large) when building some packages

I'm not 100% sure what the cause of this is, because I can't get a reproducible test case.

If I try to build postgresql-simple, this is the output from cabal (with -v3): https://gist.github.com/anonymous/bda9095cee1eda5640c0

Wondered whether it was related to hs-boot files, but I cannot reproduce the error with the basic example from http://www.haskell.org/ghc/docs/7.0.3/html/users_guide/separate-compilation.html.

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