Persona Login in my app automatically assumes error, even though the login is successful. What's wrong with my JS? :/

personaLogin.js

function personaLogin(url) {

  navigator.id.get(function(assertion) {
    if (assertion) {
      // This code will be invoked once the user has successfully
      // selected an email address they control to sign in with.
      $.ajax({
        url: '/users/sign_in',
        type: "POST",
        dataType: "json",
        cache: false,
        data: {
          "assertion": assertion
        },
        success:function(data, status){
          window.location.href = '/';
        },
        error:function(data, status){
          alert(data.statusText + ": " + data.responseText);
        }
      });
    } else {
      // something went wrong! the user isn't logged in.
      alert("Could not log you in!");
    }
  });
}