diff --git a/nemu/src/monitor/sdb/watchpoint.c b/nemu/src/monitor/sdb/watchpoint.c
index b70ce79..d283a7d 100644
--- a/nemu/src/monitor/sdb/watchpoint.c
+++ b/nemu/src/monitor/sdb/watchpoint.c
@@ -110,6 +110,9 @@ WP* get_wp(int NO) {
 
 void free_wp(WP *_wp) {
   WP* wp = del_wp(&head, _wp);
+  if (wp == NULL) {
+    printf("free_wp: no such watchpoint\n");
+  }
   wp->next = free_;
   free_ = wp;
   wp->expr[0] = '\0';