diff --git a/tools/Makefile b/tools/Makefile
index 0c1d8b64a4..9e28027835 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -159,6 +159,7 @@ qemu-traditional-recurse = \
 	set -e; \
 		$(buildmakevars2shellvars); \
 		export CONFIG_BLKTAP1=n; \
+		export BUILDING_QEMU_TRAD=y; \
 		cd qemu-xen-traditional-dir; \
 		$(1)
 
diff --git a/tools/Rules.mk b/tools/Rules.mk
index 34d495fff7..6e135387bd 100644
--- a/tools/Rules.mk
+++ b/tools/Rules.mk
@@ -141,9 +141,12 @@ endif
 
 CFLAGS_libxenlight += $(CFLAGS_libxenctrl)
 
+# Don't add -Werror if we are used by qemu-trad build system.
+ifndef BUILDING_QEMU_TRAD
 ifeq ($(CONFIG_WERROR),y)
 CFLAGS += -Werror
 endif
+endif
 
 ifeq ($(debug),y)
 # Use -Og if available, -O0 otherwise