AWS lambda function retrieving dynamodb data as a error message

I have written simple Lambda function to scan data from dynamodb, but data is getting retrieved with error message tag and lambda function shows message as execution failed

    var AWS = require('aws-sdk');
    var DOC = require("dynamodb-doc");
    var dynamo = new DOC.DynamoDB();
    exports.handler = function (event, context, callback) {

     var params = {
        TableName: "Movies",
       // ProjectionExpression: "#yr, Movie",
        FilterExpression: "#yr = :thisyear",
        ExpressionAttributeNames: {
        "#yr": "year",
    },
    ExpressionAttributeValues: {
         ":thisyear" : 2009
    }       
};

  dynamo.scan(params, function(err, data){
          if (err){
        callback("error occoured");
          }
          else{
        callback(JSON.stringify(data.Items));
          }          
  });
  };

Result

{
  "errorMessage": "[{\"year\":2009,\"Movie\":\"Jab tak hai jaan\"}]"
}

via Shan