NSURLSession data download never completes

I'm trying to download a small data file using NSURLSession, but the completion handler is never called (where "never" means I waited long after the specified timeout).

NSURLSessionConfiguration* config =
    NSURLSessionConfiguration.ephemeralSessionConfiguration;
config.timeoutIntervalForResource = 120.0;
config.timeoutIntervalForRequest = 120.0;

NSURLSession* session = [NSURLSession sessionWithConfiguration: config];

[session dataTaskWithURL: inURL
            completionHandler:
    ^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error)
    {
        ...
    }];

I'm trying to update some old code that used the deprecated NSURLConnection class, and am using the same URL, so I know that the URL is good.