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?

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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.