diff --git a/core/src/Streamly/Internal/Data/Builder.hs b/core/src/Streamly/Internal/Data/Builder.hs
index 8983eeeac..4b22f8584 100644
--- a/core/src/Streamly/Internal/Data/Builder.hs
+++ b/core/src/Streamly/Internal/Data/Builder.hs
@@ -32,7 +32,7 @@ newtype Builder s m a =
 -- | Maps a function on the output of the fold (the type @b@).
 instance Functor m => Functor (Builder s m) where
     {-# INLINE fmap #-}
-    fmap f (Builder step1) = Builder (fmap (\ ~(a, s') -> (f a, s')) . step1)
+    fmap f (Builder step1) = Builder (fmap (\ (a, s) -> (f a, s)) . step1)
 
 {-# INLINE fromPure #-}
 fromPure :: Applicative m => b -> Builder s m b