--- a/dom/media/PeerConnection.js
+++ b/dom/media/PeerConnection.js
@@ -725,18 +725,17 @@ RTCPeerConnection.prototype = {
       return this._chain(() => {
         let p = Promise.all([this.getPermission(), this._certificateReady])
           .then(() => new this._win.Promise((resolve, reject) => {
             this._onCreateOfferSuccess = resolve;
             this._onCreateOfferFailure = reject;
             this._impl.createOffer(options);
           }));
         p = this._addIdentityAssertion(p, origin);
-        return p.then(
-          sdp => new this._win.RTCSessionDescription({ type: "offer", sdp: sdp }));
+        return p.then(sdp => Cu.cloneInto({ type: "offer", sdp: sdp }, this._win));
       });
     });
   },
 
   createAnswer: function(optionsOrOnSuccess, onError) {
     // This entry-point handles both new and legacy call sig. Decipher which one
     let onSuccess, options;
     if (typeof optionsOrOnSuccess == "function") {
@@ -760,19 +759,17 @@ RTCPeerConnection.prototype = {
               throw new this._win.DOMException("No outstanding offer",
                                                "InvalidStateError");
             }
             this._onCreateAnswerSuccess = resolve;
             this._onCreateAnswerFailure = reject;
             this._impl.createAnswer();
           }));
         p = this._addIdentityAssertion(p, origin);
-        return p.then(sdp => {
-          return new this._win.RTCSessionDescription({ type: "answer", sdp: sdp });
-        });
+        return p.then(sdp => Cu.cloneInto({ type: "answer", sdp: sdp }, this._win));
       });
     });
   },
 
   getPermission: function() {
     if (this._havePermission) {
       return this._havePermission;
     }
@@ -1124,28 +1121,26 @@ RTCPeerConnection.prototype = {
 
   get localDescription() {
     this._checkClosed();
     let sdp = this._impl.localDescription;
     if (sdp.length == 0) {
       return null;
     }
 
-    return new this._win.RTCSessionDescription({ type: this._localType,
-                                                    sdp: sdp });
+    return new this._win.RTCSessionDescription({ type: this._localType, sdp });
   },
 
   get remoteDescription() {
     this._checkClosed();
     let sdp = this._impl.remoteDescription;
     if (sdp.length == 0) {
       return null;
     }
-    return new this._win.RTCSessionDescription({ type: this._remoteType,
-                                                    sdp: sdp });
+    return new this._win.RTCSessionDescription({ type: this._remoteType, sdp });
   },
 
   get peerIdentity() { return this._peerIdentity; },
   get idpLoginUrl() { return this._localIdp.idpLoginUrl; },
   get id() { return this._impl.id; },
   set id(s) { this._impl.id = s; },
   get iceGatheringState()  { return this._iceGatheringState; },
   get iceConnectionState() { return this._iceConnectionState; },