Firebase: User.photoUrl to String

I'm trying to store all the user's profile photos in the realtime database, but I'm stucked with a problem.

When trying to upload the PhotoUrl with this code, this error shows up.

Code:

var user = firebase.auth().currentUser;

user.updateProfile({

  // Here, "name" and "imgUrl" are parameters of parent function.
  // I don't think this is the reason, but if it helps,
  // "imgUrl" always come from a <img> src.
  displayName: name,
  photoUrl: imgUrl

}).then(function(){

  // Here is where I try to upload all to the database
  firebase.database().ref("/Usuarios/" + user.uid).set({
    "email": email,
    "name": name,
    "photoUri": user.photoUrl, // This is where I think the problem originates.
    "uid": user.uid  // This calls user again, and it works perfectly.
  });

});

Error:

Uncaught Error: Firebase.set failed: First argument contains undefined in property 'Usuarios.kD1tD1NAmsaGiW0zj6lFz9GDWro1.photoUri'.

When I do alert(user.photoUrl), it shows as 'undefined'.

The first thing I thought was that it can't be saved in database because it isn't a String, so I tried calling toString() from it, but I can't.

toString() error:

Uncaught TypeError: Cannot read property 'toString' of undefined

So, how can I save the photoUrl from a user (as String) in Firebase Database?

Thanks in advance.