[Help] - FormView with ImageButton does not raise validation

Hi there!
I have the following code already done in my FormView:
<asp:FormView ID="fvwInserirPlanoDiscussao" runat="server" DataSourceID="sdsMariaPlanosInserir"
OnDataBound="fvwInserirPlanoDiscussao_DataBound" DataKeyNames="OBJECTID">
<InsertItemTemplate>
Nome Plano:
<asp:TextBox ID="tbxNomePlano" runat="server" Text='<%# Bind("NOMEPLANO") %>'>
</asp:TextBox>
<asp:RequiredFieldValidator ID="rfvNomePlano" runat="server" ControlToValidate="tbxNomePlano"
ErrorMessage="Por favor, insira o nome do plano"> ...