title: DynamoDB : SET list_append not working using aws sdk
body: <p>I need to append a string to a string set in a dynamodb table using the corresponding key. This is the Update expression I use to do updateItem :</p>
  
  <pre><code>  var params = {
      "TableName" : tableName,
      "Key": {
        "ID": {
          S: "20000"
        }
      },
      "UpdateExpression" : "SET #attrName = list_append(#attrName, :attrValue)",
      "ExpressionAttributeNames" : {
        "#attrName" : "entries"
      },
      "ExpressionAttributeValues" : {
        ":attrValue" : {"SS":["000989"]}
      }   };
  </code></pre>
  
  <p>This works when I do updateItem() using the aws cli. But when using aws-sdk in nodejs, I am getting the error:</p>
  
  <pre><code>Invalid UpdateExpression: Incorrect operand type for operator or function; operator or function: list_append, operand type: M\n
  </code></pre>
  
  <p>Any help?
  Thanks</p>