CompareValidator doesnt work if the textbox is left empty

 

 
  
 I have two textboxes where the user will input integers - i need to do a validation on these textboxes:

txtStartIndex should be less than txtEndIndex - this works except with either one of the textboxes is left empty/blank
how can i fix this?
thanks


<td align="right"> <asp:TextBox ID="txtStartIndex" runat="server" Width="50px"></asp:TextBox> <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtStartIndex" ErrorMessage="*" ValidationExpression="[0-9]*" ToolTip="Numeric Values Only"></asp:RegularExpressionValidator> <td align="right"& ...