--- a/dom/media/PeerConnection.js
+++ b/dom/media/PeerConnection.js
@@ -139,23 +139,27 @@ SessionDescription.prototype = {
                                       Ci.nsIDOMRTCSessionDescription
                                     ],
                                     flags: Ci.nsIClassInfo.DOM_OBJECT}),
 
   QueryInterface: XPCOMUtils.generateQI([
     Ci.nsIDOMRTCSessionDescription, Ci.nsIDOMGlobalObjectConstructor
   ]),
 
-  constructor: function(win, type, sdp) {
+  constructor: function(win, descriptionInitDict) {
     if (this._win) {
       throw new Error("Constructor already called");
     }
     this._win = win;
-    this.type = type;
-    this.sdp = sdp;
+    if (descriptionInitDict !== undefined) {
+      this.type = descriptionInitDict.type || null;
+      this.sdp = descriptionInitDict.sdp || null;
+    } else {
+      this.type = this.sdp = null;
+    }
   },
 
   toString: function() {
     return JSON.stringify({
       type: this.type, sdp: this.sdp
     });
   }
 };