diff --git a/src/mqtt_client_wrapper.cpp b/src/mqtt_client_wrapper.cpp
index 8adc0b0..ab0c033 100644
--- a/src/mqtt_client_wrapper.cpp
+++ b/src/mqtt_client_wrapper.cpp
@@ -93,8 +93,11 @@ static void on_mqtt_message_received(const std::string &topic, const std::string
             reply["seqNo"] = seqNo;
 
             // 发送应答
-            MQTTClientWrapper::publish(g_app_config.mqtt.topics.substream_down_ack, reply.dump());
-            LOG_INFO("[MQTT] Replied to video_down: " + reply.dump());
+            if (mqtt_client)
+            {
+                mqtt_client->publish(g_app_config.mqtt.topics.substream_down_ack, reply.dump());
+                LOG_INFO("[MQTT] Replied to video_down: " + reply.dump());
+            }
         }
         else if (topic == g_app_config.mqtt.topics.substream_down)
         {