I am working on 1-1 chat app. i need to get the paginated messages in descending order of a specific chat. I don’t have any extra indexes, primary index includes a partition key chatId and a sort key createdAt. createdAt is timestamp iso 8601 format. Here is my query i run in aws appsync console

    query ListMessages {
  listMessages(chatId: "W29iamVjdCBPYmplY3Rd", limit: 50) {
    items {
      chatId
      createdAt
      body
      isSeen
      messageId
      senderId
    }
    nextToken
  }
}

it gives me this error Unsupported element '$[query][scanIndexForward]'.
Here is my resolver code where the problem arises

export function request(ctx) {
    const { chatId, limit, nextToken } = ctx.args;
    
    return {
        version: '2017-02-28',
        operation: 'Query',
        query: {
            expression: 'chatId = :chatId',
            expressionValues: {
                ':chatId': { S: chatId }
            },
            scanIndexForward: false 
        },
        limit :limit || 50,
        nextToken,
    }
    
    // return query({
    //     limit,
    //     nextToken,
    //     filter,
    // })
}

All works fine when i removes the scanIndexForward: false from my resolver code. But i need it to be false to get messages in descending order.
Thank you for your time.