Querying an index with Android Amplify GraphQL

I'm trying to perform a GraphQL query against an index using Amplify and the Android SDK.

I have a query that works when tested via the AppSync dashboard. It returns the one record I can see in DynamoDB that it should match against. membersEmailByInviteCode is an index defined in the Schema.

query get { 
  membersEmailByInviteCode(email: "test@example.com" inviteCode: { eq: "CustomCode"} ) { 
    items { 
    id 
   } 
  } 
 }

When I enable CloudWatch Logs and run the query from within the code I can see the following being sent, but I get an empty object back, which I'm guessing means that it hasn't been able to find anything.

query get($email: String!, $code: String!) { membersEmailByInviteCode(email: $email inviteCode: { eq: $code} ) { items { id } } }, Operation: null, Variables: 
{
    "email": "test@example.com",
    "code": "CustomCode"
}

Operation is null, where it is get if I run the query via the console. I can't see a way of settings this.

The Android side of things look like this

   private fun getEmailCodeRequest(email: String, code: String): GraphQLRequest<ClubMember> {

        val document = (
                "query get(\$email: String!, \$code: String!) { "
                  + "membersEmailByInviteCode(email: \$email inviteCode: { eq: \$code} ) { "
                    + "items { "
                      + "id "
                    + "} "
                  + "} "
                + "}")

        return SimpleGraphQLRequest(
            document,
            mapOf("email" to email, "code" to code),
            ClubMember::class.java,
            GsonVariablesSerializer())
    }

and

Amplify.API.query(getEmailCodeRequest(email.text.toString(), code.text.toString()),

I have been able to create a record using GraphQL, and I'm using auth and Rest API, so my app is working OK with the Android SDK. It just I can't get this to work.

Anyone got any ideas?

Thanks