diff --git a/servant-mock/src/Servant/Mock.hs b/servant-mock/src/Servant/Mock.hs
index e4437fba..7d17dca5 100644
--- a/servant-mock/src/Servant/Mock.hs
+++ b/servant-mock/src/Servant/Mock.hs
@@ -3,6 +3,7 @@
 {-# LANGUAGE FlexibleContexts    #-}
 {-# LANGUAGE FlexibleInstances   #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE PolyKinds           #-}
 {-# LANGUAGE TypeFamilies        #-}
 {-# LANGUAGE TypeOperators       #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}