Error using RequiredFieldValidator control when opening vs 15

The project started in Visual Studio 13 then updated to version 15 but at the time of testing give this error.

When I comment on the validations, it works again

<asp:RequiredFieldValidator id="rfvTnitp" runat="server"
                 ControlToValidate="TBUnitp"
                 ErrorMessage="requerido"
                 Display="Dynamic"
                 ValidationGroup="vgProduct" 
                 ForeColor="Red"> </asp:RequiredFieldValidator>