Knockout.js - passing parameters

I have a problem with Knockout.js. I want to pass username to a function and display it on alert. Something strange is happening. I get alerts each time when i refresh page with correct usernames, but after i click it i don't get any response. What is wrong here? Here is my code:

<ul data-bind="foreach: contacts">
   <li class="user-box"><span class="user-box-name" data-bind="text: username, click: $root.userClick(username)"></span>
   </li>
</ul>

and

self.userClick = function (x) {
    alert(x);
}