diff --git a/mk/config.mk.in b/mk/config.mk.in
index 3bf97e83235e47533e3da0113678ab121dbb470a..1559cd33718221efc9b92d915bc4335d206a9866 100644
--- a/mk/config.mk.in
+++ b/mk/config.mk.in
@@ -272,7 +272,7 @@ WithHappyHc = $(WithGhcHc)
 # HappyHcOpts gives the flags to pass to the Haskell compiler used
 # 	      to compile the Happy sources with.
 #
-HappyHcOpts = 
+HappyHcOpts = -O
 
 #################################################################################
 #