Code that reproduces the issue: <asp:TextBox ID="Textbox1" runat="server" ValidationGroup="vg1" /> <telerik:RadButton ID="RadButton1" runat="server" ValidationGroup="vg1" Text="click" /> <asp:RequiredFieldValidator ID="Requiredfieldvalidator1" ErrorMessage="text box is empty" ControlToValidate="Textbox1" ValidationGroup="vg1" runat="server" /> <asp:ValidationSummary ID="Validationsummary1" runat="server" ValidationGroup="vg1" ShowMessageBox="true" />