Javascript CasperJS`assertField`失败，在`fill'之后有未定义的值`

我似乎无法使这一主张通过。我已经添加了其他断言，用于验证字段的存在性和其他一些东西，试图使其正常工作，但运气不佳。我觉得我可能忽略了一些显而易见的事情

测试文件.js

var dropdownBtn     = '.btn.dropdown-toggle';
var signinForm      = '#signin-form';
var signinDropdown  = '.dropdown-menu';

casper.start(host + '/');

casper.then(function () {
    this.test.assertUrlMatch(host, 'Guest should not be redirected');
    this.test.assertExist(signinForm, 'Signin form exists');
    this.test.assertExist(dropdownBtn, 'Signin button exists');
    this.test.assertExist('[name="nickname"]', 'Nickname field exists');
    this.test.assertExist('[name="password"]', 'Password field exists');
});

casper.thenClick(dropdownBtn, function () {
    this.test.assertVisible(signinDropdown);

    this.fill(signinForm, {
        'nickname':    'bleep',
        'password':    'bloop'
    }, false);

    this.test.assertField('nickname', 'bleep'); // undefined
    this.test.assertField('password', 'bloop'); // undefined
});

casper.run(function () {
    this.test.renderResults(true);
});

login.html

<div class="dropdown-menu allow-click">
    <form method="post" action="#" id="signin-form">
        <div class="controls nickname">
            <label for="field-signin-nickname">Nickname</label>
            <input type="text" name="nickname" id="field-signin-nickname">
            <img class="maker-image">
        </div>
        <div class="controls password">
            <label for="field-signin-password">Password</label>
            <input type="password" name="password" id="field-signin-password">
        </div>
        <div class="controls pull-right">
            <input id="signin-btn" type="submit" class="btn btn-primary submit" value="Go!">
        </div>
    </form>
</div>

Test file: test/functional/test_file.js
PASS Guest should not be redirected
PASS Signin form exists
PASS Signin button exists
PASS Nickname field exists
PASS Password field exists
PASS Selector is visible

FAIL "nickname" input field has the value "bleep"
#    type: assertField
#    subject: false
#    inputName: "nickname"
#    actual: null
#    expected: "bleep"
FAIL "password" input field has the value "bloop"
#    type: assertField
#    subject: false
#    inputName: "password"
#    actual: null
#    expected: "bloop"
FAIL 8 tests executed in 10.252s, 6 passed, 2 failed.

Details for the 2 failed tests:

In test/functional/session_signin.js:0
   assertField: "nickname" input field has the value "bleep"
In test/functional/session_signin.js:0
   assertField: "password" input field has the value "bloop"