diff --git a/src/main/java/com/github/nkzawa/socketio/client/Socket.java b/src/main/java/com/github/nkzawa/socketio/client/Socket.java
index 36291a6..f917bf1 100644
--- a/src/main/java/com/github/nkzawa/socketio/client/Socket.java
+++ b/src/main/java/com/github/nkzawa/socketio/client/Socket.java
@@ -344,9 +344,13 @@ public class Socket extends Emitter {
     }
 
     private void onack(Packet<JSONArray> packet) {
-        logger.fine(String.format("calling ack %s with %s", packet.id, packet.data));
         Ack fn = this.acks.remove(packet.id);
-        fn.call(toArray(packet.data));
+        if (fn != null) {
+            logger.fine(String.format("calling ack %s with %s", packet.id, packet.data));
+            fn.call(toArray(packet.data));
+        } else {
+            logger.fine(String.format("bad ack %s", packet.id));
+        }
     }
 
     private void onconnect() {