Modal Popup Validation

I have 2 required field validators in an update panel in my modal popup panel. I use Matt Gibson's Validator dll and tag mapping for validators inside update panels. However, in this modal popup, postback is not prevented. As you can see, I give the modal popup no control over the save button. Yes, the validation group is the same on the validators and the button. Yes, the validation appears onblur. With the errormessage displayed, if I click on the save button, the postback goes through. How do I fix this?

<ajaxToolkit:ModalPopupExtender runat="server" ID="mpe_fix"BackgroundCssClass="modalBackground"CancelControlID="btn_Cancel"PopupControlID="pnl_fix"TargetControlID="hdn_dummy" /><asp:Button runat="server" ID="btn_save"Text="Save"ValidationGroup="fix"CausesValidation="true" />

Bump.