range validator always invalid?

hi, i have the ffg scenario: a textbox with 3 validators. It works fine without the rangevalidator. The moment i add in the range validator, and i try to update the page, the error "MyTextBox must be greater than 0" is shown. If i try to edit the textbox it still shows the error. Only if i clear the textbox and retype the value does the error msg disappear. This only happens when updating. Adding is fine. I've checked the html when running and the textbox does have its value set to a valid number, but i still get this annoying error. Any ideas why? urgent-please help

<asp:TextBox runat="server"  ID="MyTextBox" Width="100px" />

<asp:RequiredFieldValidator ID="RequiredFieldValidator3" ControlToValidate="MyTextBox"
            runat="server" ErrorMessage="MyTextBox is required" Display="Dy ...