UINavigationBar back button does not appear

Question

I have a navigationBar, with a UIImage on it's title, like this:

self.navigationItem.titleView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"top_logo.png"]];

When i select a row, the "back" button does not appear. Why?

I have the exact same code on other's viewControllers, and it appears. I don't understand why...

Thanks,

RL