Require field validator not working properly for Ajax FilteredTexboxExtender

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C#

Ajax

Hi..

In my page i have a textbox for which i am using Ajax Filtered texbox Extender to filter digits,a Require field validator to validate that textbox and a Submit button to submit the page.
The problem is When i am clicking the submit button required field validator is firing good as well as page is also going to code behind, i mean require field validator not stoping page to postback. validation is firing and next second page is postbacking.

Please help me.. Thanks advance.

<asp:TextBox ID="txtpostalcode" runat="server" CssClass="multiline_textbox"MaxLength="6" Width="300px" TabIndex="3" onkeyup="Count(this.id)"></asp:TextBox>
                                                                            <ajax:FilteredTextBoxExtender ID="ftAddress1Zipcode" FilterType="Numbers" TargetControlID="txtpostalcode"runat="server">

<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="*"ControlToValidate="txtpostalcode"></asp:RequiredFieldValidator>