diff --git a/src/ai/ai_client.c b/src/ai/ai_client.c
index 5fc2ac7a..ffa3947d 100644
--- a/src/ai/ai_client.c
+++ b/src/ai/ai_client.c
@@ -90,7 +90,10 @@ _aiwin_validate(gpointer user_data)
         return NULL;
     }
 
-    if (!wins_ai_exists((ProfAiWin*)user_data)) {
+    pthread_mutex_lock(&lock);
+    gboolean win_exists = wins_ai_exists((ProfAiWin*)user_data);
+    pthread_mutex_unlock(&lock);
+    if (!win_exists) {
         log_warning("[AI-THREAD] aiwin=%p no longer exists — window was closed", (void*)user_data);
         return NULL;
     }