AJAX :: Timer OnTick Event Not Being Triggered?

I am trying to modify the Interval and Enabled properties via ViewState but it is not working.
Here is my code:

protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
//set the default Refresh Interval to 3 seconds and enable Timer to TRUE
ViewState["TimerEnable"] = true;
ViewState["RefreshInterval"] = 3000;
}
Markup:
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Timer ID="Timer1" OnTick="Timer1_Tick" runat="server"
Enabled = <%# Convert.ToBoolean(ViewState["TimerEnable"].ToString()) %>
Interval = <%# Convert.ToInt32(ViewState["RefreshInterval"].ToString()) %> >
</asp:Timer>
...
If I use Enabled = "true" and Interval = 3000, the code works just fine!