Why doesn't keyboard show when I make a UITextField within a UIAlertView become the first responder?

I have an alert view that pops up with a text field inside it, and I want the text field to become the first responder and show the keyboard immediately.

When I use [summaryField becomeFirstResponder]; I get a blinking cursor in the text field, but the keyboard does not pop up. However, if I don't make the text the first responder, and instead just click inside of it when the alert shows, the keyboard does pop up.

Any ideas on how a text field can become the first responder without showing the keyboard? I would guess it has something to do with the text field being added as a subview to the alert view, but I don't know what exactly the problem is. My code is below, without the "becomeFirstResponder" call.

UIAlertView *titleAlert = [[UIAlertView alloc] initWithTitle:@"Enter title" message:@" "     
    delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
summaryField = [[UITextField alloc] initWithFrame:CGRectMake(20.0, 45.0, 245.0, 25.0开发者_StackOverflow中文版)];
[summaryField setBackgroundColor:[UIColor whiteColor]];
[titleAlert addSubview:summaryField];
[titleAlert show];
[titleAlert release];
[summaryField release];

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try [textField becomeFirstResponder] in UIAlertViewDelegate willPresentAlertView: method.