DynamoDB query with lambda (node.js) FilterExpression using contains

Here is the table structure. In posts column it is a list of strings
So I been reading and trying to use filterExpression with 'contains'. What documentation said in node.js just need to use a CONTAINS b where a can be a list and b for me is a string I am searching for.

var db = new doc.DynamoDB();

var params = {
    TableName: "WIT_Search",
    KeyConditionExpression: "#type = :tag and #key between :start AND :end",
    FilterExpression: "#tag contains :post",
    ExpressionAttributeNames: {
        "#type": "type",
        "#tag": "posts",
        "#key": "key",
    },
    ExpressionAttributeValues: {
        ":tag": "tag",
         ":start": tag+"_",
         ":end": tag+"_999",
         ":post": postID
    }
};

console.log(params);

db.query(params, function(err, data) {
    if (err) {
        console.error("Unable to query. Error:", JSON.stringify(err, null, 2));
    } else {
        console.log("Query succeeded.");
        console.log('Count =',data.Items.length);

        if (data.Items.length > 0){
            console.log("post exists in tag");
        }else{
            console.log("post doesnt exist in tag");
        }
    }
});

Whenever I do testing it gives me this message:

  Unable to query. Error: {
      "message": "Invalid FilterExpression: Syntax error; token: \"contains\", near: \"#tag contains (\"",
      "code": "ValidationException",
    }

The weird thing on AWS Console it works without the problem, just using in lambda with node.js it gives this error.