Node.js and mysql Callback: request in request callback

All I want to do is insert some data if my database does not have this, so I insert Insert SQL into my Select SQL callback function, but I got this error:

{[Error: Unable to execute Query after calling quit.]: "PROTOCOL_ENQUEUE_AFTER_QUIT", fatal: false}

my code snippet is here:

db.query('SELECT count(*) as Resultcount FROM tablename WHERE email = ? and password = ?', [post.email, post.password], function(error, result){
    if (result[0].Resultcount == 0){
        var query2 = db.query('INSERT INTO tablename SET ?', [post], function(err, result) {
            if(err){
              console.log(err);
           }
             console.log(result);
          });
    }
    else{
        console.log('have data already');
    }
});

Can someone give me some advice? thank

---- update ----

in fact, the select SQL callback function is not an anonymous function, my db.end () code snippet looks like this:

var QueryResults = new queryResultFuntion(Back_results);

    db.query('SELECT count(*) as Resultcount FROM tablename WHERE email = ? and password = ?', [post.email, post.password], QueryResults.queryResult );


    db.end();