diff --git a/rules.mk b/rules.mk
index eb8c3c5..da9c243 100644
--- a/rules.mk
+++ b/rules.mk
@@ -6,7 +6,7 @@ ifneq ($(USE_EVENTFLAG),)
 CSRC += $(CHOPSTX)/eventflag.c
 endif
 
-INCDIR = $(CHOPSTX)
+INCDIR _= $(CHOPSTX)
 
 BUILDDIR = build
 OUTFILES = $(BUILDDIR)/$(PROJECT).elf $(BUILDDIR)/$(PROJECT).bin