I'm following the "Your Second iOS App" and I decided to play with the code to understand Objective C well...

What I'm trying to do is simply adding an object to a mutable array in a class. Here are the classes:

BirdSighting.h

#import <Foundation/Foundation.h>

@interface BirdSighting : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSString *location;
@property (nonatomic, copy) NSDate *date;

-(id) initWithName: (NSString *) name location:(NSString *) location date:(NSDate *) date;
@end

BirdSighting.m

#import "BirdSighting.h"

@implementation BirdSighting
-(id) initWithName:(NSString *)name location:(NSString *)location date:(NSDate *)date
{
    self = [super init];

    if(self) {
        _name = name;
        _location = location;
        _date = date;

        return self;
    }

    return nil;
}

@end

BirdSightingDataController.h

#import <Foundation/Foundation.h>
@class BirdSighting;

@interface BirdSightingDataController : NSObject
@property (nonatomic, copy) NSMutableArray *masterBirdSightingList;

- (NSUInteger) countOfList;
- (BirdSighting *) objectInListAtIndex: (NSUInteger) theIndex;
- (void) addBirdSightingWithSighting: (BirdSighting *) sighting;
@end

BirdSightingDataController.m

#import "BirdSightingDataController.h"

@implementation BirdSightingDataController
- (id) init {
    if(self = [super init]) {
        NSMutableArray *sightingList = [[NSMutableArray alloc] init];
        self.masterBirdSightingList = sightingList;
        return self;
    }

    return nil;
}

-(NSUInteger) countOfList 
{
    return [self.masterBirdSightingList count];
}

- (BirdSighting *) objectInListAtIndex: (NSUInteger) theIndex
{
    return [self.masterBirdSightingList objectAtIndex:theIndex];
}

- (void) addBirdSightingWithSighting: (BirdSighting *) sighting
{
    [self.masterBirdSightingList addObject:sighting];
}
@end

And this is where I'm trying to add a BirdSighting instance to the mutable array:

#import "BirdsMasterViewController.h"
#import "BirdsDetailViewController.h"

#import "BirdSightingDataController.h"
#import "BirdSighting.h"

@implementation BirdsMasterViewController

- (void)awakeFromNib
{
    [super awakeFromNib];

    BirdSightingDataController *dataController = [[BirdSightingDataController alloc] init];

    NSDate *date = [NSDate date];
    BirdSighting *sighting = [[[BirdSighting alloc] init] initWithName:@"Ebabil" location:@"Ankara" date: date];

    [dataController addBirdSightingWithSighting: sighting];
    NSLog(@"dataController: %@", dataController.masterBirdSightingList);

    self.dataController = dataController;

}
..........
@end

It throws NSInvalidArgumentException in BirdSightingDataController addBirdSightingWithSighting method...

What am I doing wrong?