focus()

O que seria esse erro?

Uncaught TypeError: campos[0].focus is not a function
    at HTMLFormElement.<anonymous>

Meu Código

var campos = [
    document.querySelector('#data'),
    document.querySelector('#quantidade'),
    document.querySelector('#valor')
];

console.log(campos);

var tbody = document.querySelector('table tbody');


document.querySelector('.form').addEventListener('submit', function(event) {

    event.preventDefault();

    var tr = document.createElement('tr');

    campos.forEach(function (campo) {

        var td = document.createElement('td');

        td.textContent = campo.value;
        tr.appendChild(td);
    });

    var tdVolume = document.createElement('td');
    tdVolume.textContent = campos[1].value * campos[2].value;

    tr.appendChild(tdVolume);

    tbody.appendChild(tr);

    campos[0] = '';
    campos[1] = 1;
    campos[2] = 0;

    campos[0].focus();


});

Erro no Chrome