set in dynamo db not properly incrementing nodejs lamnda function

1var AWS = require('aws-sdk');
2var documentClient = new AWS.DynamoDB.DocumentClient({'region': 'eu-central-1'});
3
4exports.handler = function(item, context, callback) {
5
6    var params = {
7        TableName: "my-table",
8        Key:{
9            "id": item.id
10        },
11        UpdateExpression: "ADD iteration :iteration SET itemdata = :itemdata",
12        ExpressionAttributeValues:{
13            ':iteration': 1,
14            ':itemdata' : item.data
15        },
16        ReturnValues:"NONE"
17};
18
19documentClient.update(params, function(err, data) {
20    if (err) {
21      console.log("Error", err);
22      const errResponse = {
23        statusCode: 500,
24        headers: {
25          "Access-Control-Allow-Origin": "*"
26        },
27        body: JSON.stringify({ Error: 500, device : "DynamoDB", detail : err })
28      };
29      callback(null, errResponse);
30    } else {
31      console.log("Success", params.Items);
32      const response = {
33        statusCode: 200,
34        headers: {
35          "Access-Control-Allow-Origin": "*"
36        },
37        body: JSON.stringify("upsert complete.")
38      };
39      callback(null, response);
40    }
41  });
42};