diff --git a/ghc/lib/exts/LazyST.lhs b/ghc/lib/exts/LazyST.lhs
index 29c7814cc6b45f82e83f1855027fc5ac976d862a..7ba30741f72c95d5b517eb02c28a36f995e427d9 100644
--- a/ghc/lib/exts/LazyST.lhs
+++ b/ghc/lib/exts/LazyST.lhs
@@ -60,7 +60,7 @@ instance Monad (ST s) where
            in
            k_a new_s
 
--- ToDo: un-inline this, it could cause problems...
+{-# NOINLINE runST #-}
 runST :: (All s => ST s a) -> a
 runST st = case st of ST st -> let (r,_) = st (PrelST.S# realWorld#) in r
 \end{code}