[Mono-list] Index is less than 0 Error

[Daniel Soto]

Hello.

I encountered an error. In an aspx page, when i click two times in a 
button, shows me this error.

/
//Index is less than 0 or more than or equal to the list count. 
Parameter name: index 1///

My page have a custom validators (appears as occurs with any)

<asp:TextBox ID="txtNumDocumento" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" 
ControlToValidate="txtNumDocumento" ErrorMessage="Debe ingresar el 
documento" ValidationGroup="Grupo1">(*)</asp:RequiredFieldValidator>
<asp:CustomValidator runat="server" ID="cusvDocumento" 
ControlToValidate="txtNumDocumento"                                      
    ErrorMessage="El documento o factura no existe" 
ValidationGroup="Grupo1" Forecolor="red" 
OnServerValidate="ValidarFactura">(*)</asp:CustomValidator>

when not passes the validation (any validator) at click, the 
<asp:button> that fires the validation, works fine. But at second click, 
appears the error message that i mentioned above.

The CustomValidator submit the page to the server, with an 
Onservervalidate event, to assign true or false to args.IsValid parameter.

Anybody can help with this error?

Thanks.