为什么下面这段代码的completionHandler没有调用？

session=[NSURLSession sharedSession];
NSURLRequest* request=[NSURLRequest requestWithURL:url];
request.HTTPMethod=@"POST";

NSURLSessionTask* task=[session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    //not called
    if (!error) {
        //
    }
}];
[task resume];