diff --git a/src/PubSubClient.h b/src/PubSubClient.h
index c70d9fd3..6d34e5a2 100755
--- a/src/PubSubClient.h
+++ b/src/PubSubClient.h
@@ -96,7 +96,7 @@ class PubSubClient : public Print {
    unsigned long lastOutActivity;
    unsigned long lastInActivity;
    bool pingOutstanding;
-   MQTT_CALLBACK_SIGNATURE;
+   MQTT_CALLBACK_SIGNATURE = NULL;
    uint32_t readPacket(uint8_t*);
    boolean readByte(uint8_t * result);
    boolean readByte(uint8_t * result, uint16_t * index);