promise and generators

function fetchBla(id) {
    return new Promise(...)
} // Do a query with given ID

function *fetchBlas() {
    var index = 1;
    while (true)
        index++;
        yield fetchBla(index);
}

let blas = fetchBlas();
blas.next().value.then(...) // Get's stuck in a loop forever .. shouldn't this return a Promise object?..
blas.next().value.then(...) // Blow's up with .then() beeing undefined