diff --git a/Distribution/Client/Sandbox.hs b/Distribution/Client/Sandbox.hs
index 66b415d..6aa71bb 100644
--- a/Distribution/Client/Sandbox.hs
+++ b/Distribution/Client/Sandbox.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE RankNTypes #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Distribution.Client.Sandbox