Problem with masked edit control

I have used masked edit control of Ajax .Net control toolkit. I am facing an issue which makes the textbox useless. I have the following code snippet:

   <tr>
           <td><cc1:MaskedEditExtender ID="MaskedEditExtender2" runat="server" TargetControlID="TextBox4"
                        Mask="C{13}"
                        MaskType="None"
                        Filtered="$%231"
                        MessageValidatorTip="true"
                        OnFocusCssClass="MaskedEditFocus"
                        OnInvalidCssClass="MaskedEditError"
                        AutoComplete="false"
                        InputDirection="LeftToRight"
                        AcceptNegative="None"
                        
                        ErrorTooltipEnabled="True"/>
                   
                </td>
                <td>
                    <asp:TextBox ID="TextBox4" runat="server"></asp:TextBox>
                </td>
                <td>
                <cc1:MaskedEditValidator ID="MaskedEditValidator4" runat="server"
                    ControlExtender="MaskedEditExtender2" ControlToValidate="TextBox4"
                    Display="Dynamic"
                        InvalidValueMessage="Please enter a custom character.">
                        </cc1:MaskedEditValidator>
                </td>
            </tr>

Suppose i have entered 123123 and now I want to make it 33123123 then i will go to the first character and simply type 3 and 3. But this makes it 333123 i.e., instead of inserting new characters it updates the old characters.