R – Error setting property on CLLocation subclass

I've defined this subclass of CLLocation

MyLocation.h

@interface MyLocation: CLLocation {
    NSString *datum;
    NSString *ellipsoid;
}

@property(nonatomic,retain) NSString *ellipsoid;
@property(nonatomic,retain) NSString *datum;

MyLocation.m

@synthesize datum,ellipsoid;

Now i get a CLLocation instance through the location manager delegate

• (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation{
  //self.myPosition is a MyLocation myPosition;
  self.myPosition = newLocation;
  [myPosition setDatum: @"WGS 84"];
  [myPosition setEllipsoid: @"WGS 84"];
  }

When i do the self.myPosition = newLocation i get an UNCAUGHT EXCEPTION and it dies

I've also tried this way with same results:

self.myPosition = (MyLocation *)newLocation;