cabal sdist trouble with GHC from head

Does anybody have a clue what's going wrong at the sdist step here? https://travis-ci.org/haskell-opengl/OpenGLRaw/jobs/38707011#L104 This only happens with a GHC from head, a build with GHC 7.8.3 is fine: https://travis-ci.org/haskell-opengl/OpenGLRaw/jobs/38707010 Any help highly appreciated... Cheers, S.