diff --git a/wled00/mqtt.cpp b/wled00/mqtt.cpp
index 15cb0c5df..8e6f2e360 100644
--- a/wled00/mqtt.cpp
+++ b/wled00/mqtt.cpp
@@ -68,7 +68,7 @@ void onMqttMessage(char* topic, char* payload, AsyncMqttClientMessageProperties
   if (strncmp(topic, mqttDeviceTopic, topicPrefixLen) == 0) {
       topic += topicPrefixLen;
   } else {
-      size_t topic_prefix_len = strlen(mqttGroupTopic);
+      topicPrefixLen = strlen(mqttGroupTopic);
       if (strncmp(topic, mqttGroupTopic, topicPrefixLen) == 0) {
           topic += topicPrefixLen;
       } else {