I am getting the page invalid at server side for below as

I have used select option control in asp.net and binded data using Ajax get api.Also i am performing the validation using RequiredFieldValidator .When i select option and try to continue button its going to server and getting page is invalid .

                    <asp:RequiredFieldValidator ID="rfvtxtWorkshops" ValidationGroup="vStudentCourses"
                                                ControlToValidate="ddlWorkshopsList" ErrorMessage="Provide Select workshop"
                                                CssClass="form-error" runat="server" Display="Dynamic" SetFocusOnError="true" />

How to fix this issue.