How can I display a UIBarButtonItem without a border?

In IB I can select a UIBarButtonItem, set it to Plain, Custom and apply an Image and it looks fine in a UIToolbar or UINavigationBar.

When I try to do this in code, I get a border:

backButton = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"arrowBack"] 
                                                   style:UIBarButtonItemStylePlain 
                                                  target:self 
                                                  action:@selector(backButtonClicked:)];
 

How can I get a UIBarButtonItem without a border? I want it to be Plain and Custom

---