Error with GridView

Hi.

I have a WebForm with a single form with a buttons the accept and cancel. Then, I have a GridView to display info related with de before form. And to the end, I have a webusercontrol to used to ModalPopupExtender to display messages related with the event to the buttons.

If the GridView is empy or this isn`t initialized, the webForm work good. But, if the GridView have rows the webForm show this error:

0x800a139e - javascript runtime error: Sys.InvalidOperationException: Two components with the same id 'MainContent_wucMensajes_mpeMensaje' can't be added to the application.

mpeMensaje is the ModalPopupExtender and wucMensajes is the WebUserControl. I use Visual Studio 2012.

Thank you.