--- a/dom/media/PeerConnection.js
+++ b/dom/media/PeerConnection.js
@@ -637,16 +637,21 @@ class RTCPeerConnection {
         throw new this._win.DOMException(msg + " - missing urls", "InvalidAccessError");
       }
       urls.map(url => nicerNewURI(url)).forEach(({ scheme, spec }) => {
         if (scheme in { turn: 1, turns: 1 }) {
           if (username == undefined) {
             throw new this._win.DOMException(msg + " - missing username: " + spec,
                                              "InvalidAccessError");
           }
+          if (username.length > 512) {
+            throw new this._win.DOMException(msg +
+                                             " - username longer then 512 bytes: "
+                                             + username, "InvalidAccessError");
+          }
           if (credential == undefined) {
             throw new this._win.DOMException(msg + " - missing credential: " + spec,
                                              "InvalidAccessError");
           }
           if (credentialType != "password") {
             this.logWarning("RTCConfiguration TURN credentialType \"" +
                             credentialType +
                             "\" is not yet implemented. Treating as password." +