Why there is a line on the top of UIImagePickerView?

I am new here and I may ask my fist stupid question now...:)

I use UIImagePickerView to take my photos but strangely there is a white line on the top of the camera...

What i have done is:

1. A new UIviewController, and the controller will present a new UIImagePickerController in viewWillAppear method:

   UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];
   imagePicker.delegate = self;
   [self presentModalViewController:imagePicker animated:YES];
   

2. I add this control into the UITabController as one of the item.

3. open my app 开发者_如何转开发and click on the tab to show UIImagePickerView. Then an unexpected line shown on the top of my camera.

Who can tell why this happen? Thanks in advance!