MaskedEditExtender and textbox placeholder

Hi !

I'm trying to use a textbox with placeholder and maskededit

When the field lost focus empty, the placeholder text is nos shown.

What to do ?

See my code:

<asp:TextBox ID="edtEmpresaCNPJ" runat="server" Width="400px" placeholder="CNPJ" /><ajaxToolkit:MaskedEditExtender ID="MaskedEditExtender1"
                                runat="server"
                                TargetControlID="edtEmpresaCNPJ"
                                Mask="99.999.999/9999-99"
                                MaskType="None" ClearMaskOnLostFocus="true"/>

Regards