Spot the error

Person.m

@interface Person : NSObject

@property (nonatomic, readonly, copy) NSString *first;
@property (nonatomic, readonly, copy) NSString *last;
@property (nonatomic, readonly, copy) NSDate *birthdate;

@end

@implementation Person

- (instancetype)initWithFirst:(NSString *)first last:(NSString *)last birthdate:(NSDate *)birthdate {
    self = [super init];
    if (!self)
        return nil;
    
    _first = [first copy];
    _last = [last copy];
    _birthdate = [birthdate copy];
    
    return self;
}

- (NSUInteger)hash {
    return (_first.hash ^ _last.hash ^ _birthdate.hash);
}

- (BOOL)isEqual:(Person *)object {
    if (self == object)
        return YES;
    
    if (![object isKindOfClass:[self class]])
        return NO;
    
    return ([_first isEqual:object.first],
            [_last isEqual:object.last] &&
            [_birthdate isEqual:object.birthdate]);
}

@end

Result.md

NSDate *date = [NSDate date];
Person *cosmo = [[Person alloc] initWithFirst:@"Cosmo" last:@"Kramer" birthdate:date];
Person *conrad = [[Person alloc] initWithFirst:@"Conrad" last:@"Kramer" birthdate:date];
BOOL equal = [cosmo isEqual:conrad];
NSLog(@"%@ and %@ are %@equal!", cosmo.first, conrad.first, (equal ? @"" : @"not "));

Cosmo and Conrad are equal!

[_first isEqual:object.first], => [_first isEqual:object.first] &&