diff --git a/Assistant/Upgrade.hs b/Assistant/Upgrade.hs
index 6efd750414..db6f6a26e0 100644
--- a/Assistant/Upgrade.hs
+++ b/Assistant/Upgrade.hs
@@ -163,7 +163,7 @@ upgradeToDistribution newdir cleanup distributionfile = do
 	{- OS X uses a dmg, so mount it, and copy the contents into place. -}
 	unpack = liftIO $ do
 		olddir <- oldVersionLocation
-		withTmpDirIn (parentDir newdir) "git-annex.upgrade" $ \tmpdir -> do
+		withTmpDirIn (fromRawFilePath (parentDir (toRawFilePath newdir))) "git-annex.upgrade" $ \tmpdir -> do
 			void $ boolSystem "hdiutil"
 				[ Param "attach", File distributionfile
 				, Param "-mountpoint", File tmpdir