Truncated incorrect double value in mysql

Question

I am trying to add strings with a separator ',' in the UPDATE statement in a column as follows:

    const sql="UPDATE Events SET rejectedBy=rejectedBy+? WHERE id=2"
    db.query(sql,[request.params.name+','],(err,result)=>{
      if(err) throw err;
    })

But I am getting error like:

    Truncated incorrect DOUBLE value: 'berlin7,'

where berlin7 is the value of request.params.name.

My table column is

    rejectedBy varchar(250) NOT NULL DEFAULT ''

How do I solve it?