Changing back button in iOS 7 disables swipe to navigate back

Changing back button in iOS 7 disables swipe to navigate back

I have an iOS 7 app where I am setting a custom back button like this:
UIImage *backButtonImage = [UIImage imageNamed:@"back-button"];
UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
[backButton setImage:backButtonImage forState:UIControlStateNormal];
backButton.frame = CGRectMake(0, 0, 20, 20);
[backButton addTarget:self
action:@selector(popViewController)
forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *backBarButtonItem = [[UIBarButtonItem alloc]
initWithCustomView:backButton];
viewController.navigationItem.leftBarButtonItem = backBarButtonItem;
But this disables the iOS 7 "swipe left to right" gesture to navigate to
the previous controller. Does anyone know how I can set a custom button
and still keep this gesture enabled?