diff --git a/src/mod/languages/mod_spidermonkey/sm.mak b/src/mod/languages/mod_spidermonkey/sm.mak
index ef56bb615f..2cce2703be 100644
--- a/src/mod/languages/mod_spidermonkey/sm.mak
+++ b/src/mod/languages/mod_spidermonkey/sm.mak
@@ -1,2 +1,2 @@
-CFLAGS += -I$(PREFIX)/include/js -I$(PREFIX)/include/nspr -DXP_UNIX -I../mod_spidermonkey
+CFLAGS += -I$(PREFIX)/include/js -I$(PREFIX)/include/nspr -DXP_UNIX -I../mod_spidermonkey -DJS_THREADSAFE
 LDFLAGS += -lnspr4 -ljs -lcurl