ajaxToolkit:FilteredTextBoxExtender fails in Internet Explorer 8

<div>
  <label>City:<span class="mandatory small">*</span></label>
  <asp:TextBox ID="txt_city"  runat="server" MaxLength="68" TabIndex="7" CssClass="text">    
  </asp:TextBox>
  <ajaxToolkit:FilteredTextBoxExtender ID="txtCityExtender" runat="server"     FilterType="LowercaseLetters,UppercaseLetters,Custom" ValidChars=" " TargetControlID="txt_city"></ajaxToolkit:FilteredTextBoxExtender>
</开发者_JAVA百科div>

Version: AjaxControlToolkit 1.0

When I try to enter number, in FF 3.6 its not allowing but in IE 8 it allows.

Any insights?