MaskedEditExtender and ClearTextOnInvalid="true"

I have a MaskedEditExtender as follows:

<asp:TextBox ID="txt_Test" Width="95%" runat="server"></asp:TextBox>
     <AJAX:MaskedEditExtender ID="mee_Test" runat="server" TargetControlID="txt_Test" Mask="9999999999"
     MaskType="Number" ClearTextOnInvalid="true" AutoComplete="false">
</AJAX:MaskedEditExtender>

It works fine as far as it only allow the user to enter numbers, but if for example they enter 123 then tab out I was expecting that the textbox would be cleared as the data entered did not match the mask, but it doesn't, it allows 123 to be entered, the question I have is this the expected behaviour, as I would have expected the textbox to be cleared, rather than allowing data that does not match the mask.