diff --git a/Makefile.am b/Makefile.am
index c0d81297145ce0a2c99ce97f08d8b1c47d301a12..90d51690e6d52a1bb674a314113b8a017cd38097 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -361,9 +361,8 @@ HS_ALL_PROGS = $(HS_PROGS) htools/test htools/hpc-htools htools/hconfd
 HS_PROG_SRCS = $(patsubst %,%.hs,$(HS_ALL_PROGS))
 HS_BUILT_TEST_HELPERS = $(HS_BIN_ROLES:%=test/%) test/hail
 
-# we don't add -Werror by default
-HFLAGS = -O -Wall -fwarn-monomorphism-restriction -fwarn-tabs -ihtools
-# extra flags that can be overriden on the command line
+HFLAGS = -O -Wall -Werror -fwarn-monomorphism-restriction -fwarn-tabs -ihtools
+# extra flags that can be overriden on the command line (e.g. -Wwarn, etc.)
 HEXTRA =
 # internal extra flags (used for htools/test mainly)
 HEXTRA_INT =