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`?