Casper asserts an element doesn't exist

[A. Mathieu]

I have an element inside a list and CasperJS doesn't find it.

Here's what I do :

this.echo(this.getHTML('div#filters > ul > li:nth-child(4)', true));
this.test.assertExists('div#filters > ul > li:nth-child(4) span#dateFilterSelector');

Outputs :

<li>
                        <fieldset class="label">
                            <span class="cursorTitle">&nbsp;</span>
                            <label>
                                Period
                            </label>
                        </fieldset>
                        <fieldset>
                            <span id="dateFilterSelector" style="cursor: pointer; ">April 23, 2013 00:00 - May 23, 2013 00:00</span>
                        </fieldset>
                    </li>
FAIL Found an element matching: div#filters > ul > li:nth-child(4) fieldset span#dateFilterSelector
#    type: assertExists
#    subject: false
#    selector: "div#filters > ul > li:nth-child(4) fieldset span#dateFilterSelector"

As you can see, the first line of code allows me to see the "span#dateFilterSelector" does exist but still, the assert doesn't find it.

Any idea ?