[web2py] TypeError: is not JSON serializable

I'm trying to list the db.table's content using plugin_wiki.widget('jqgrid',table='table') but despite I put in controller, view o in a wiki_page (with respective sintaxis for each) I always get "TypeError: is not JSON serializable". My db model was generated by the app_wizard. I have looked for

[web2py] TypeError: is not JSON serializable

I have a generic controller to show the contents of a table in a grid. For one of the 33 tables I get this error while the other seems to work without problem: Controller: @auth.requires_login() def show_data(): tabel = request.vars.tabel.lower() data= plugin_wiki.widget('jqgrid',t