diff --git a/puncher.nim b/puncher.nim
index 11a6562..abddddb 100644
--- a/puncher.nim
+++ b/puncher.nim
@@ -70,6 +70,8 @@ proc handleMsg*(puncher: Puncher, msg: string, peerIp: IpAddress,
                 peerPort: Port) =
   ## Handles an incoming UDP message which may complete the Futures returned by
   ## ``initiate`` and ``respond``.
+  if msg != "ACK":
+    return
   let (_, myPort) = puncher.sock.getLocalAddr()
   let query = Attempt(srcPort: myPort, dstIp: peerIp, dstPorts: @[peerPort])
   let i = puncher.attempts.find(query)