diff --git a/make/ise.mk b/make/ise.mk
index 3feede5..18a951f 100644
--- a/make/ise.mk
+++ b/make/ise.mk
@@ -9,7 +9,7 @@ bit: $(PRJ).bit
 prom : $(PRJ).mcs
 timing : $(PRJ).twr
 
-NETLIST_DIR += ./
+NETLIST_DIR ?= ./
 
 # Netlist
 $(PRJ).ngc: $(DIRS) $(PREQ) ./project/$(PRJ).ucf