why [response expectedContentLength] always return -1

-(void) connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse    *)response {
    [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
    if([recievedData length]) [ recievedData setLength:0 ];

    download_size =[response expectedContentLength];
}

I have this code. download_size is NSInteger. expectedContentLenght always return: -1. Maybe someone know why? I tried use long, but effect was the same.

Thanks for help.