Why Html.Checkbox(“Visible”) returns “true, false” in ASP.NET MVC 2?

I'm using Html.Checkbox("Visible") for displaying a check box to user. In post back, FormCollection["Visible"] value is "true, false". Why?

in view:

<td>                
    <%: Html.CheckBox("Visible") %>
</td>

in controller:

 adslService.Visible = bool.Parse(collection["Visible"]);