diff --git a/Git/UpdateIndex.hs b/Git/UpdateIndex.hs
index 5d07e20112..76fe0cce91 100644
--- a/Git/UpdateIndex.hs
+++ b/Git/UpdateIndex.hs
@@ -44,6 +44,7 @@ streamUpdateIndex repo as = pipeWrite params repo $ \h -> do
 	streamer h s = do
 		hPutStr h s
 		hPutStr h "\0"
+		hFlush h
 
 {- A streamer that adds the current tree for a ref. Useful for eg, copying
  - and modifying branches. -}