Objective-c – [NSManagedObject sayHello]: unrecognized selector sent to instance 0x

I try to extend NSManagedObject.
Using XCode I created MyBox.m and MyBox.h (directly from the xcdatamodel file).

Then I modified these files:

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>


@interface MyBox : NSManagedObject

@property (nonatomic, retain) NSDate * endDate;
@property (nonatomic, retain) NSNumber * globalId;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSDate * startDate;

-(NSString *)sayHello;

@end

and

#import "MyBox.h"
@implementation MyBox

@dynamic endDate;
@dynamic globalId;
@dynamic name;
@dynamic startDate;

-(NSString *)sayHello {
    return @"hello";
}  

@end

I can fetch all myBoxes

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"MyBox" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

NSMutableArray *myBoxes = [context executeFetchRequest:fetchRequest error:&error];

but later I call

MyBox *myBox = [myBoxes objectAtIndex:indexPath.row];    
    [myBox sayHello];

it compiles but then I get

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSManagedObject sayHello]: unrecognized selector sent to instance 0x8e73fc0'

If I only read a value like

NSLog(@"%@", myBox.name);

it works

I found similar problems here, but no solution.
Thanks for your help.