diff --git a/ch4/os/Makefile b/ch4/os/Makefile
index 04340cd..834beb3 100644
--- a/ch4/os/Makefile
+++ b/ch4/os/Makefile
@@ -46,5 +46,5 @@ clean:
 	rm -rf ./target* && rm -rf ./src/link_app.S
 
 kill:
-	pkill -9 qemu
+	-pkill -9 qemu