UIDatePicker not rendering properly for certain modes in UIPopoverController

I have a UIDatePicker using UIDatePickerModeTime that I use in an iPhone app modally, works fine:

Using this same basic code for iPad, but putting the view in a UIPopoverController gives some pretty inconsistent results. My desired mode (UIDatePickerModeTime) plus the other three modes are shown below. Notice how 2 of them are not re开发者_如何学Cndering at all, but instead have no content

UIDatePickerModeTime

UIDatePickerModeCountDownTimer

UIDatePickerModeDateAndTime

UIDatePickerModeDate

What could be causing the UIDatePickerModeTime and UIDatePickerModeDateAndTime modes to go haywire?

Instantiating the UIDatePicker with CGRectZero was the culprit. Even though the code worked on iPhone, and I call sizeToFit explicitly on it to set the size, changing the CGRectZero to CGRectMake(0,0,320,160) was what got it to display. Go figure.