Graphql_flutter runMutation's QueryResult is returning null, though the mutation is executing correctly

Question

I'm using graphql_flutter 3.1.0 and have started the authentication for my app and am needing to set the user in my Provider state on successful creation. My mutation to CreateUser is currently:

final String createUserMutation = """
mutation CreateUser(\$firstName: String!, \$username: String!, \$phoneNumber: String!) {
  CreateUser(firstName: \$firstName, username: \$username, phoneNumber: \$phoneNumber, role: USER) {
    id,
    firstName,
    username,
    phoneNumber,
    role
  }
}
""";

This successfully creates the user in my Neo4j database. If I run that same query in my GraphQL interface, I get the response exactly as expected:

{
  "data": {
    "CreateUser": {
      "id": "e2a44525-fd46-4aae-8e00-38d263b769e6",
      "firstName": "Sam",
      "username": "sgamgee",
      "phoneNumber": "3216543216",
      "role": "USER"
    }
  }
}

In my SignupForm widget I am wrapping the form in Mutation and passing the createUserMutation to the MutationOptions like so:

class SignupFormState extends State<SignupForm> {
  final _formKey = GlobalKey<FormState>();

  final user = User();

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(35),
      child: Mutation(
        options: MutationOptions(
          documentNode: gql(createUserMutation)
        ),
        builder: (RunMutation runMutation, QueryResult result) {
          return UserCreateForm(
            formKey: _formKey, 
            user: user,
            runMutation: runMutation,
            result: result
          );
        },
      )
    );
  }
}

All is well so far, no errors. When I attempt to submit the form via an onPressed closure:

onPressed: () async {
  final form = _formKey.currentState;
  if (form.validate()) {
    form.save();
                    
    runMutation({
      'firstName': user.firstName,
      'username': user.username,
      'phoneNumber': user.phoneNumber
    });
                    
    if (result.hasException) {
      return AlertBox(
        type: AlertType.error,
        text: result.exception.toString()
      );
    } else if (result.loading) {
      return const Center(
        child: CircularProgressIndicator(),
      );
    } else {
      // NOTE: result.data is null though the mutation is running successfully. 
      var createdUser = result.data['CreateUser'];
      Provider.of<UserProvider>(context, listen: false).signUp(createdUser);
                      
      if (Provider.of<UserProvider>(context, listen: false).isAuthenticated()) {
        Navigator.pushReplacementNamed(context, '/signup-success');
      } else {
        Navigator.pushReplacementNamed(context, '/signup');
      }
    }
  }
}

The result.data is consistently null. I know neither runMutation or result are futures and thus cannot be awaited. Also hasException and loading are present and false or true as the case may be. Does anyone notice anything else going on that could cause result.data to consistently be null?