--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -2571,17 +2571,17 @@ GeckoDriver.prototype._checkIfAlertIsPre
  * session.
  */
 GeckoDriver.prototype.quitApplication = function(cmd, resp) {
   if (this.appName != "Firefox") {
     throw new WebDriverError("In app initiated quit only supported in Firefox");
   }
 
   let flags = Ci.nsIAppStartup.eAttemptQuit;
-  for (let k of cmd.parameters.flags) {
+  for (let k of cmd.parameters.flags || []) {
     flags |= Ci.nsIAppStartup[k];
   }
 
   this.stopSignal_();
   resp.send();
 
   this.sessionTearDown();
   Services.startup.quit(flags);