WebDriverException launching w3c spec driver

Dart Version: 2.4.0
OS: MacOS Mojave
webdriver: 2.1.1

When attempting to launch a w3c spec webdriver, the following exception is encountered:
```
00:00 Given that I am testing the createDriver function when I create a w3c spec driver [E]
WebDriverException (500):
package:webdriver/src/handler/w3c/utils.dart 109:9 parseW3cResponse
package:webdriver/src/handler/w3c/session.dart 19:21 W3cSessionHandler.parseCreateResponse
package:webdriver/src/common/request_client.dart 46:25 SyncRequestClient.send
package:webdriver/sync_core.dart 63:26 createDriver
package:webdriver/sync_io.dart 35:5 createDriver
test/createDriver_test.dart 7:23 main..
```

Code that results in the exception:
```
import 'package:test/test.dart';
import 'package:webdriver/sync_io.dart';

void main() {
group('Given that I am testing the createDriver function', () {
test('when I create a w3c spec driver', () {
final driver = createDriver(spec: WebDriverSpec.W3c);
});
});
}
```

Am I missing something in my setup here, or is this a bug? There is literally "" in the exception so this is hard to tell what's going wrong.