CustomValidator problem

I am trying to validate listbox by CustomValidator control using client script but the clien script function is not fired!
Here is my code, what I am doin wrong?

<asp:listbox id="Departments" runat="server" Width="160px" SelectionMode="Multiple"></asp:listbox>
<asp:CustomValidator id="Departmentsvld" runat="server" ControlToValidate="Departments"  ClientValidationFunction="HasDepartment" Text="*" />

<script>                  
function HasDepartment(source, arguments)
      {
            alert("Fired");
            for(var i=0; i < document.getElementById('Departments').length; i++)
            {
                  if(document.getElementById("Departments")[i].selected)
                  {
                        arguments.IsValid = true;
                        return;
                        //this is selected
                  }
            }
            arguments.IsValid = false;
      }
</script>