diff --git a/netwerk/cache/filecache/Makefile.in b/netwerk/cache/filecache/Makefile.in
index 06a60ebc3895..c729a3e4ee8a 100644
--- a/netwerk/cache/filecache/Makefile.in
+++ b/netwerk/cache/filecache/Makefile.in
@@ -56,5 +56,9 @@ EXTRA_LIBS	= $(NSPR_LIBS)
 override NO_SHARED_LIB=1
 override NO_STATIC_LIB=
 
+ifeq ($(OS_ARCH),HP-UX)
+OS_CXXFLAGS += +u1
+endif
+
 include $(topsrcdir)/config/rules.mk