Why AjaxMaskExtender doesn't work with backspace ?

I have used AJAX latest tool kit but AjaxMaskExtender doesn't work properly i.e. BackSpace doesn't in it. why ? 

<div class="form-group"><label class="control-label" for="disabledInput">CNIC</label><asp:TextBox ID="TextBoxCNIC" runat="server" MaxLength="15" Height="40px"
                                    CssClass="form-control"></asp:TextBox><asp:RequiredFieldValidator ID="RequiredFieldValidatorModel" runat="server" ControlToValidate="TextBoxCNIC"
                                    ErrorMessage="*" Display="Dynamic" ValidationGroup="0"></asp:RequiredFieldValidator><cc1:MaskedEditExtender ID="MaskedEditExtenderCNIC" runat="server" TargetControlID="TextBoxCNIC"
                                    Mask="99999-9999999-9"
                                    MessageValidatorTip="true"
                                    OnFocusCssClass="MaskedEditFocus"
                                    OnInvalidCssClass="MaskedEditError"
                                    MaskType="Number"
                                    InputDirection="LeftToRight"
                                    ErrorTooltipEnabled="True"></cc1:MaskedEditExtender></div>