diff --git a/ghc/compiler/utils/FastString.lhs b/ghc/compiler/utils/FastString.lhs
index 0cbb3f8945d542b81463fb75b84f0ad308796b5b..92afb6874061892dc7285a576c9b950c2c66e42a 100644
--- a/ghc/compiler/utils/FastString.lhs
+++ b/ghc/compiler/utils/FastString.lhs
@@ -137,7 +137,7 @@ unpackFS :: FastString -> String
 #if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ <= 205
 unpackFS (FastString _ l# ba#) = byteArrayToString (_ByteArray (0,I# l#) ba#)
 #else
-unpackFS (FastString _ l# ba#) = unpackCStringBA# l# ba#
+unpackFS (FastString _ l# ba#) = unpackCStringBA# ba# l#
 #endif
 unpackFS (CharStr addr len#) =
  unpack 0#