--- a/testing/marionette/message.js
+++ b/testing/marionette/message.js
@@ -252,17 +252,17 @@ this.Response = class {
    * @throws {Error}
    *     If the {@code error} is not a WebDriverError, the error is
    *     propagated.
    */
   sendError(err) {
     let wd = error.isWebDriverError(err);
     let we = wd ? err : new WebDriverError(err.message);
 
-    this.error = error.toJson(err);
+    this.error = error.toJson(we);
     this.body = null;
     this.send();
 
     // propagate errors that are implementation problems
     if (!wd) {
       throw err;
     }
   }