Cant Display AlertView or PopOver

Question

I am using cloudmade map for displaying map even in offline mode. My issue is I am not able to display UIPopoverController in map view. Also I tried to display alert view over it. It does not displayed. I tried this code.

MyViewController *mvc=[[MyViewController alloc]initWithNibName:@"MyViewController" bundle:[NSBundle mainBundle]];
UIPopoverController *popover=[[UIPopoverController alloc]initWithContentViewController:mvc];


NSLog(@"%f",marker.frame.size.width);
[popover presentPopoverFromRect:marker.frame inView:mapView permittedArrowDirections:UIPopoverArrowDirectionUp animated:YES];

Thanks in advance.