Query AWS DynamoDB Global Secondary Indexes

I am trying to query a global secondary index to see if a value exists. I am getting the error

Query condition missed key schema element: Username

I am querying using

AWSDynamoDBObjectMapper *dynamoDBObjectMapper = [AWSDynamoDBObjectMapper defaultDynamoDBObjectMapper];
    AWSDynamoDBQueryExpression *queryExpression = [AWSDynamoDBQueryExpression new];
    queryExpression.hashKeyValues = textfieldUsername;
    queryExpression.scanIndexForward = [NSNumber numberWithBool:YES];
    queryExpression.limit = [NSNumber numberWithInt:1];
    queryExpression.indexName = @"Username-index"; //using indexTable for query

    [[dynamoDBObjectMapper query:[UsersDB class] expression:queryExpression] continueWithBlock:^id(BFTask *task) {


        if (task.error) {
            NSLog(@"Error: [%@]", task.error);

        } else {

            AWSDynamoDBPaginatedOutput *paginatedOutput;

            paginatedOutput = task.result;

            NSLog(@"pagin %@", paginatedOutput);


        }

        return nil;

    }];

I was going to use LOAD instead of query but I understand it only searches the primary hash key and not the global one? I did make an additional database class with the secondary global index as the hash but this produced a key element does not match schema error.

What am I doing wrong?

Username is a global secondary index