diff --git a/lib/protocol.js b/lib/protocol.js
index b660973..46725f1 100644
--- a/lib/protocol.js
+++ b/lib/protocol.js
@@ -1163,6 +1163,7 @@ LongWriter.prototype.write = function(buffer, offset) {
 
 function get(packetId, toServer) {
   var packetInfo = packets[packetId];
+  assert.ok(packetInfo, "unrecognized packet id: " + packetId);
   return Array.isArray(packetInfo) ?
     packetInfo :
     toServer ?