You have put a validation on an ASP.NET page. The trouble is if you hit cancel, the validation control stills take effect. This can be cumberson because you are just canceling an action. The easy cure is put the following attributes in the cancel button
<asp:Button id="Button1" runat="server" Text="Cancel" CausesValidation="False"> </asp:Button>
This should fix it. When you click Cancel button, validation will not take place.