Asp.net regular expression problem

I have a regular expression  like below what my goal when user changed dropdown country in that  ddl country selected index depending on the country i have setted

VatRegularExpression = sp.CountryVatRegularExpressionGet(ddlCountry.SelectedValue)  ---here i  am getting regular expression from database

If Not String.IsNullOrEmpty(VatRegularExpression) Then
RegtxtVatCode.Enabled = True
RegtxtVatCode.ValidationExpression = VatRegularExpression
End If

<asp:RegularExpressionValidator ID="RegtxtVatCode" Enabled="false"
runat="server" ControlToValidate="txtVatCode" Display="Dynamic" CssClass="help-block"
ErrorMessage="<%$ Resources: res, WrongVatCode %>">
</asp:RegularExpressionValidator>

But my problem is when there is no VatRegularExpression value. for example when i select country germany there is no regular expression for this country.then also still it is giving the error message but it should not give the error message.Can you help me out please