How to get data from Core Data in PickerView? iOS

There is an application that has a list of employees and a list of orders. And when you create an order, a PickerView should appear in the TextField field with a list of employees (with a name). I implement it like this:

- (void)viewDidLoad { NSManagedObjectContext *managedObjectContext = [self managedObjectContext]; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Employees"]; self.empl=[[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy]; UIPickerView *picker = [[UIPickerView alloc]init]; picker.dataSource = self; picker.delegate = self; [picker setShowsSelectionIndicator:YES]; [self.empTaskField setInputView:picker]; } #pragma mark - UIPickerView DataSource Method - (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView{ return 1; } - (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component{ return self->_empl.count; } #pragma mark - UIPickerView Delegate Method -(NSString *) pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component{ return [_empl objectAtIndex:row]; } 

When you click on TextField error appears:

2016-03-27 23: 32: 57.177 Employee Task [2730: 150090] - [Employees copyWithZone:]: unrecognized selector 0x7fcf2ac84fd0 2016-03-27 23: 32: 57.513 Employee Task [2730: 150090] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '- [Employees copyWithZone:]: unrecognized selector sent to instance 0x7fcf2ac84fd0'

How to solve this problem?