diff --git a/src/fetch/client.h b/src/fetch/client.h
index f0a4868..ce93a3b 100644
--- a/src/fetch/client.h
+++ b/src/fetch/client.h
@@ -340,21 +340,33 @@ Client *focustop(Monitor *m) {
 }
 
 Client *get_next_stack_client(Client *c, bool reverse) {
+	if (!c || !c->mon)
+		return NULL; // 添加输入检查
+
 	Client *next = NULL;
 	if (reverse) {
 		wl_list_for_each_reverse(next, &c->link, link) {
+			if (!next)
+				continue; // 安全检查
+
 			if (c->mon->has_visible_fullscreen_client && !next->isfloating &&
 				!next->isfullscreen)
 				continue;
-			if (VISIBLEON(next, c->mon) && next != c)
+
+			// 添加更安全的 VISIBLEON 检查
+			if (next != c && next->mon && VISIBLEON(next, c->mon))
 				return next;
 		}
 	} else {
 		wl_list_for_each(next, &c->link, link) {
+			if (!next)
+				continue; // 安全检查
+
 			if (c->mon->has_visible_fullscreen_client && !next->isfloating &&
 				!next->isfullscreen)
 				continue;
-			if (VISIBLEON(next, c->mon) && next != c)
+
+			if (next != c && next->mon && VISIBLEON(next, c->mon))
 				return next;
 		}
 	}
diff --git a/src/mango.c b/src/mango.c
index 33c49a6..26e3167 100644
--- a/src/mango.c
+++ b/src/mango.c
@@ -101,7 +101,7 @@
 	(A && !(A)->isfloating && !(A)->isminied && !(A)->iskilling &&             \
 	 !(A)->ismaximizescreen && !(A)->isfullscreen)
 #define VISIBLEON(C, M)                                                        \
-	((M) && (C)->mon == (M) && ((C)->tags & (M)->tagset[(M)->seltags]))
+	((C) && (M) && (C)->mon == (M) && ((C)->tags & (M)->tagset[(M)->seltags]))
 #define LENGTH(X) (sizeof X / sizeof X[0])
 #define END(A) ((A) + LENGTH(A))
 #define TAGMASK ((1 << LENGTH(tags)) - 1)