--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -635,20 +635,20 @@ GeckoDriver.prototype.setSessionCapabili
     }
 
     for (let key in from) {
       switch (key) {
         case "desiredCapabilities":
           to = copy(from[key], to);
           break;
         case "requiredCapabilities":
-          if (from[key]["proxy"]) {
-              this.setUpProxy(from[key]["proxy"]);
-              to["proxy"] = from[key]["proxy"];
-              delete from[key]["proxy"];
+          if (from[key].proxy) {
+              this.setUpProxy(from[key].proxy);
+              to.proxy = from[key].proxy;
+              delete from[key].proxy;
           }
           for (let caps in from[key]) {
             if (from[key][caps] !== this.sessionCapabilities[caps]) {
               errors.push(from[key][caps] + " does not equal " +
                   this.sessionCapabilities[caps])   ;
             }
           }
           break;