diff --git a/lib/Connection.js b/lib/Connection.js
index 40e817a..4b33bb4 100644
--- a/lib/Connection.js
+++ b/lib/Connection.js
@@ -942,7 +942,7 @@ Connection.prototype._resUntagged = function(info) {
     if (this.state === 'connected' && !this._curReq) {
       clearTimeout(this._tmrConn);
       var err = new Error('Received negative welcome: ' + info.text);
-      err.level = 'protocol';
+      err.source = 'protocol';
       this.emit('error', err);
       this._sock.end();
     }