initWithContentsOfURL is nil even with NSAppTransportSecurity set to NSAllowsArbitraryLoads

Question

This line of code gives me an NSData object with nil contents:

NSData *jsonData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:urlString]];

If I look at urlString (which uses http rather than https) in Xcode it's giving me a URL which, in a browser, gives me valid JSON.

For testing I have set this in the plist:

<key>NSAppTransportSecurity</key>
<dict>
    <key>NSAllowsArbitraryLoads</key>
    <true/>
</dict>

However, I am still getting an NSData object with nil contents. If it helps it was all working before the NSAppTransportSecurity requirement which is why I tried setting NSAllowsArbitraryLoads.

Any suggestions what else I can try?

==== UPDATE

    NSError* error = nil;
    NSData *jsonData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:urlString] options:nil error:&error];
    NSLog(@"%@", [error localizedDescription]);

gives (null) in the console.