Core Data fetchedProperty issue on iOS 7

it has been few days that I am struggling with core Data. My application works very well on iOS 6, when I use iOS 7 on the Simulator I got this error:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', 
    reason: '*** -constantValue only defined for abstract class. 
    Define -[NSKeyPathExpression constantValue]!'

I use fetchedproperties (n2) in my DataModel and I retriev them as :

UIImage *image2 = [UIImage imageNamed:[NSString stringWithFormat:@"%@.png",
    [(_maqam.n2)[0] valueForKey:@"noteImgPath"]]];
[B2 setImage:image2 forState:UIControlStateNormal];

Thank you in advance for any help.