diff --git a/mutable-containers/test/Spec.hs b/mutable-containers/test/Spec.hs
index fc0d5660..53cb6cc3 100644
--- a/mutable-containers/test/Spec.hs
+++ b/mutable-containers/test/Spec.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE FlexibleContexts #-}
+
 import Control.Monad             (forM_)
 import Data.Mutable
 import Data.Sequence             (Seq)