Confuse with MaskEditValidator

.NET
1

I drag and drop a maskedit control and here’s the mark-up


<cc1:MaskedEditValidator ID="MaskedEditValidator" runat="server" ControlExtender="mDateEntry"
                        ControlToValidate="txtEntryDate" EmptyValueMessage="Date is required." InvalidValueMessage="Invalid date entry."
                        TooltipMessage="YYYY-MM-DD" ValidationGroup="SaveEntry"></cc1:MaskedEditValidator>

And for my MaskEditExtender


<cc1:MaskedEditExtender ID="mDateEntry" runat="server" Century="2000" ErrorTooltipEnabled="True"
        Mask="9999-99-99" MaskType="Date" TargetControlID="txtEntryDate" UserDateFormat="YearMonthDay" InputDirection="RightToLeft">
    </cc1:MaskedEditExtender>

But then, even I inputted a valid date, still the maskeditvalidator’s InvalidValueMessage triggers. Any idea please?