graphql-如何从索引中查询两个值？Appsync 查询 Dynamodb

我的解析器模板

{
   "version" : "2017-02-28",
    "operation" : "Query",
    "query" : {
        "expression" : "receiverusername = :receiverusername and createdat > :createdat",
        "expressionValues" : {
            ":receiverusername" : $util.dynamodb.toDynamoDBJson($context.identity.username),
            ":createdat" : $util.dynamodb.toDynamoDBJson($ctx.args.input.lastDate),
        }
    },
    "index" : "receiverusername-createdat-index",
}

我正在尝试查询日期和用户名，但我一直收到这个错误

{
  "data": {
    "listMyMessages": null
  },
  "errors": [
    {
      "path": [
        "listMyMessages"
      ],
      "data": null,
      "errorType": "DynamoDB:DynamoDbException",
      "errorInfo": null,
      "locations": [
        {
          "line": 2,
          "column": 3,
          "sourceName": null
        }
      ],
      "message": "One or more parameter values were invalid: Condition parameter type does not match schema type (Service: DynamoDb, Status Code: 400, Request ID: 7I18ODEK46H52NMBSF99OTNSQ7VV4KQNSO5AEMVJF66Q9ASUAAJG)"
    }
  ]

我该如何查询？

我试图从文档中学习，但没有给我任何线索！