Paste layer Facebook API SDK JS

I'm trying to grab the Facebook cover of the logged in user, but I'm not having success.

FB.login(function(response){
if(response.authResponse){
    FB.api('/me', function(fb){
        var email   = fb.email;
        var nasc    = fb.birthday;
        var nome    = fb.name;
        var genero  = fb.gender;
        var local   = fb.hometown;
        var idUser  = fb.id;

        FB.api('/'+idUser+'?fields=source', function(resp){
            console.log(resp);
            return false;
        });

I'm doing as it is in the Facebook documentation, but it doesn't work out.

Object {error: Object}

error: Object

code: 100

message: "(#100) Tried accessing nonexisting field (source) on node type (User)"

type: "OAuthException"

I got it. I put cover in the parameter. But it's not in Doc. Or I read wrong.

FB.api('/'+idUser+'/?fields=cover', function(resp){
    console.log(resp);
    return false;
});