TypeError: val.toString is not a function - mysql npm

Question

I use the package 'mysql' with NPM on a nodeJS backend and I don't understand the error message :

TypeError: val.toString is not a function at Object.escape (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/sqlstring/lib/SqlString.js:52:33) at Object.objectToValues (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/sqlstring/lib/SqlString.js:180:89) at Object.escape (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/sqlstring/lib/SqlString.js:54:26) at Object.format (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/sqlstring/lib/SqlString.js:100:19) at Connection.format (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/mysql/lib/Connection.js:271:20) at Connection.query (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/mysql/lib/Connection.js:189:22) at Function.create (/Applications/MAMP/htdocs/nodeJS_livredor/models/message.js:7:19) at /Applications/MAMP/htdocs/nodeJS_livredor/server.js:41:17 at Layer.handle [as handle_request] (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/express/lib/router/layer.js:95:5) at next (/Applications/MAMP/htdocs/nodeJS_livredor/node_modules/express/lib/router/route.js:137:13)

My code :

let mysql = require('mysql');

let connexion = mysql.createConnection({
    host        : 'localhost',
    user        : 'root',
    password    : 'root',
    database    : 'livredor'
});

console.log('avant connexion.connect');
connexion.connect((err) => {
    if (err) {
        console.error(`Erreur de connexion: ${err.stack}`);
        return;
      }
      console.log(`Connecté`);
});

module.exports = connexion;

I see both console.log in my console, so I guess it's come from 'connexion.connect' but I don't know why.

If someone can help, ty :)