Ajax Control Toolkit MaskedEdit - Optionnal char

Hi,

What do you put for an optionnal numnber? I have a textbox whick takes up to 2 digits. When I entered a number less than 10 I get an error becuase it is expecting 2 digits with the following configuration :

<asp:MaskedEditExtender runat="server"
                                TargetControlID="SegmentsTextBox"
                                Mask="99"
                                MessageValidatorTip="true"
                                OnFocusCssClass="MaskedEditFocus"
                                OnInvalidCssClass="MaskedEditError"
                                MaskType="Number"
                                InputDirection="RightToLeft"
                                AcceptNegative="None"
                                ErrorTooltipEnabled="True"/>