Custom back button in UINavigationController

For an application I'm developing, I need to display a custom back button in a navigation bar. I have the button asset as a PNG image, and I'm writing this code:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
    backButton.frame = CGRectMake(0, 0, 79, 29.0);
    [backButton setImage:[UIImage imageNamed:@"button_back.png"] forState:UIControlStateNormal];
    self.navigationItem.backBarButtonItem = [[[UIBarButtonItem alloc] initWithCustomView:backButton] autorelease];

}

When I push this view controller, the custom button does not show up, and instead I get the standard back button with the title of this view controller inside.

Things I already tried:

1. Doubled check that the button backButton is created properly, by adding it to the view hierarchy. It displays properly.
2. In the same method, changed the title property of the navigationItem and confirmed that it changes (as expected) the content of my back button.

Can anyone spot what I'm doing wrong? Did anyone succeed in using a custom image as the back button on with a UINavigationController?