Master recently broke on OS X

Hello Simon, It looks like one of the patches that you pushed to master today may have broken the build on OS X. According to Harbormaster something in the range of f148513ccd93..7129861397f8 caused T5611 to fail on the OS X build bot [1]. Could you have a look? Cheers, - Ben [1] https://phabricator.haskell.org/harbormaster/build/14220/?l=100