--- a/testing/marionette/proxy.js
+++ b/testing/marionette/proxy.js
@@ -166,17 +166,18 @@ proxy.AsyncMessageChannel = class {
    */
   reply(uuid, obj = undefined) {
     // TODO(ato): Eventually the uuid will be hidden in the dispatcher
     // in listener, and passing it explicitly to this function will be
     // unnecessary.
     if (typeof obj == "undefined") {
       this.sendReply_(uuid, proxy.AsyncMessageChannel.ReplyType.Ok);
     } else if (error.isError(obj)) {
-      let serr = error.toJson(obj);
+      let err = error.wrap(obj);
+      let serr = error.toJson(err);
       this.sendReply_(uuid, proxy.AsyncMessageChannel.ReplyType.Error, serr);
     } else {
       this.sendReply_(uuid, proxy.AsyncMessageChannel.ReplyType.Value, obj);
     }
   }
 
   sendReply_(uuid, type, data = undefined) {
     let path = proxy.AsyncMessageChannel.makePath(uuid);