diff --git a/src/Streamly/Internal/Data/Stream/IsStream/Expand.hs b/src/Streamly/Internal/Data/Stream/IsStream/Expand.hs
index 998e1c587..72b6f9aab 100644
--- a/src/Streamly/Internal/Data/Stream/IsStream/Expand.hs
+++ b/src/Streamly/Internal/Data/Stream/IsStream/Expand.hs
@@ -707,7 +707,8 @@ zipAsyncWith f = zipAsyncWithM (\a b -> return (f a b))
 --
 -- @since 0.6.0
 {-# INLINE mergeBy #-}
-mergeBy :: IsStream t => (a -> a -> Ordering) -> t m a -> t m a -> t m a
+mergeBy ::
+       (IsStream t, Monad m) => (a -> a -> Ordering) -> t m a -> t m a -> t m a
 mergeBy f m1 m2 = fromStream $ K.mergeBy f (toStream m1) (toStream m2)
 
 -- | Like 'mergeBy' but with a monadic comparison function.