diff --git a/conf/Makefile b/conf/Makefile
index be823d40..3b65ad1e 100644
--- a/conf/Makefile
+++ b/conf/Makefile
@@ -23,7 +23,7 @@ $(o)cf-parse.tab.c: $(o)cf-parse.y
 	$(BISON) $(BISON_DEBUG) $(BISONFLAGS) -dv -pcf_ -b $(@:.tab.c=) $<
 
 $(o)cf-lex.c: $(s)cf-lex.l
-	$(FLEX) $(FLEX_DEBUG) -s -B -8 -Pcf_ -o$@ $<
+	$(FLEX) $(FLEX_DEBUG) -f -s -B -8 -Pcf_ -o$@ $<
 
 $(o)cf-lex.o: $(o)cf-parse.tab.h $(o)keywords.h
 $(o)cf-lex.o: CFLAGS+=-Wno-sign-compare -Wno-unused-function