Try
NSMutableArray* _board;
...
_board = [[NSMutableArray alloc] init];
for (int i = 0; i < 8; i++) {
NSMutableArray *row = [[NSMutableArray alloc] init];
for (int j = 0; j < 8; j++) {
[row addObject:[NSNull null]];
}
[_board addObject:row];
}
Reason for using [NSNull null]
instead of nil
can be seen here