diff --git a/xpcom/build/Makefile.in b/xpcom/build/Makefile.in
index f429f3d3dd8..aab955b0f4a 100644
--- a/xpcom/build/Makefile.in
+++ b/xpcom/build/Makefile.in
@@ -70,7 +70,7 @@ CPPSRCS		= \
 		nsXPCOMStrings.cpp \
 		$(NULL)
 
-ifdef MOZ_ENABLE_LIBXUL
+ifndef MOZ_ENABLE_LIBXUL
 ifeq (,$(filter-out WINNT WINCE OS2,$(OS_ARCH)))
 CPPSRCS += dlldeps.cpp
 endif