working with maskedEditValidator and maskeditextender

i have a textbox which is bind to a masktext and a validator.
when i load the page without the validator. i received no errors and everything works find.
if i bind a masktextbox to a validator, i cant load the page and received a error saying start index cannot be less than zero.

i have few dropdown list control in the same page as well. but none of this is giving me problem
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager> <asp:TextBox ID="itxt" runat="server"></asp:TextBox> <asp:MaskedEditExtender ID="IPmask" runat="server" TargetControlID="itxt" ErrorTooltipEnabled="True" Mask="___.___.___.___" MaskType="Number" AutoCompleteValue="1" CultureName="en-SG"> </asp:MaskedEdit ...