diff --git a/nemu/tools/gen-expr/Makefile b/nemu/tools/gen-expr/Makefile
index 28955a617e48bf6e276f83388df769edff58cf0b..b697c928bedcaff59d16d21aec24b17a31d03c81 100644
--- a/nemu/tools/gen-expr/Makefile
+++ b/nemu/tools/gen-expr/Makefile
@@ -5,4 +5,4 @@ $(APP): gen-expr.cc
 
 .PHONY: clean
 clean:
-	-rm $(APP) .code.c .expr
+	-rm -f $(APP) .code.c .expr