Why is map caching not working?

Question

Objective-C

- (NSURL *)URLForTilePath:(MKTileOverlayPath)path {
    return [NSURL URLWithString:[NSString stringWithFormat:@"http://tile.site.ru/%d/%d/%d.png", path.z, path.x, path.y]];
}

- (void)loadTileAtPath:(MKTileOverlayPath)path
                result:(void (^)(NSData *data, NSError *error))result
{
    if (!result) {
        return;
    }

    NSData *cachedData = [self.tileCache objectForKey:;

    if (cachedData) {
        NSLog(@"Self Tile Cache Data: %@", cachedData);
        result(cachedData, nil);
    } else {
        NSURLRequest *request = [NSURLRequest requestWithURL:[self URLForTilePath:path]];
        [[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            [self.tileCache setObject:data forKey:;
            result(data, error);
        }] resume];
    }
}

for some reason the cache is not being captured