Firebase cloud functions - TypeError: Can not read property '

I explain my problem, I have made a function for firebase for when it is inserted, in this case, a new side in Firebase Firestone , the function sends a notification to the users. The problem is when I try to get the id of the document because I need it. I have followed the examples that come in the documentation and as I do I get this error when trying to read the id:

  

TypeError: Can not read property 'side' of undefined       at exports.bandoCreado.functions.firestore.document.onCreate (/user_code/index.js:17:30)       at Object. (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:112:27)       at next (native)       at /user_code/node_modules/firebase-functions/lib/cloud-functions.js:28:71       at __awaiter (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:24:12)       at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:82:36)       at /var/tmp/worker/worker.js:700:26       at process._tickDomainCallback (internal / process / next_tick.js: 135: 7)

What comes from this line in particular:

const idBando=event.params.bando;

I leave the full function in case there is still some doubt, a greeting.

 exports.bandoCreado = functions.firestore
.document('bandos/{bando}')
.onCreate((event) =>{
    const idBando=event.params.bando;

    var datos=event.data.data();

    var titulon=datos.titulo;
    var contenidon=datos.contenido;
    var f_empiecen=datos.f_empiece;
    var f_finn=datos.f_fin;
    var t_stampn=datos.t_stamp;

    const payload={
        data: {
            titulo: titulon,
            mensaje: contenidon,
            id: idBando,
            f_empiece: f_empiecen,
            f_fin: f_finn,
            t_stamp: t_stampn
        }
    };

    return admin.messaging().sendToTopic("notificaciones",payload)
        .then(function(response) {
        // See the MessagingTopicResponse reference documentation for the
        // contents of response.
        console.log("Successfully sent message:", response);
        return "";
      })
      .catch(function(error) {
        console.log("Error sending message:", error);
        return "";
      });
});