Uncaught Error FirebaseError: Function CollectionReference.doc() cannot be called with an empty path

here show: var cod = document.getElementById(“codigo”).value;————- I get an error when I want to pass the cod variable of the document.getElementById — doc(cod); show error.– How do I make the query in firestore?

Uncaught Error FirebaseError: Function CollectionReference.doc() cannot be called with an empty path.
    at Br (d:Documentosapp web 2firebasejsfirestoredistprebuilt.js:184:9)
    at Xd (d:Documentosapp web 2firebasejsfirestoredistprebuilt.js:14495:19)
    at gp (d:Documentosapp web 2firebasejsfirestoredistprebuilt.js:14886:44)
    at im.doc (d:Documentosapp web 2firebasejsfirestoredistprebuilt.js:18010:79)
    at mostrarFila (d:Documentosapp web 2ventas1ventas1.js:24:44)
    at onclick (d:Documentosapp web 2ventas1ventas1.html:49:105)
Br @ d:Documentosapp web 2firebasejsfirestoredistprebuilt.js:184:9

var db = firebase.firestore();

var cod = document.getElementById("codigo").value;
//para que me muestre la tabla del tbody todas las filas con sus columanas
var tabla =document.getElementById("tabla");

function mostrarFila(){
    var docRef = db.collection("Producto").doc(cod);/* aca está el error*/

tabla.innerHTML= "";

docRef.get().then((doc) => {
    if (doc.exists) {
        console.log("Document data:", doc.data().codigo);

    } else {
        // doc.data() will be undefined in this case
        console.log("No such document!");
    }
}).catch((error) => {
    console.log("Error getting document:", error);
});
}