diff --git a/src/PubSubClient.h b/src/PubSubClient.h
index be4bd67..13ab692 100755
--- a/src/PubSubClient.h
+++ b/src/PubSubClient.h
@@ -73,7 +73,7 @@
 #define MQTTQOS1        (1 << 1)
 #define MQTTQOS2        (2 << 1)
 
-#ifdef ESP8266
+#if defined(ESP8266) || defined(ESP32)
 #include <functional>
 #define MQTT_CALLBACK_SIGNATURE std::function<void(char*, uint8_t*, unsigned int)> callback
 #else