[web2py] Problems to exclude the field "id" from the jqgrid (using plugin_wiki)

[Lisandro]

I have the following model:
    db.define_table('ciudadanos', Field('tipo'),
Field('nro_documento'), Field('valor'))


In the view, the following line shows the jqgrid with all the fields
of the table (including "id" field):
    {{=plugin_wiki.widget('jqgrid', db.ciudadanos)}}
The jqgrid shows fine.

Then, I add the "fields" argument using any subset of fields, for
example:
 * {{=plugin_wiki.widget('jqgrid', db.ciudadanos, fields='id,
nro_documento')}}
 * {{=plugin_wiki.widget('jqgrid', db.ciudadanos, fields='id, tipo,
valor')}}
 * {{=plugin_wiki.widget('jqgrid', db.ciudadanos, fields='id,
valor')}}
Notice that "id" field is always present.

But when I exclude the "id" field from the "fields" argument, then no
data is shown in the jqgrid. For example:
 * {{=plugin_wiki.widget('jqgrid', db.ciudadanos, fields='tipo,
nro_documento')}}
 * {{=plugin_wiki.widget('jqgrid', db.ciudadanos, fields='valor')}}
In this cases the jqgrid doesn't show data.

¿Is it extrictly necessary to include "id" field in jqgrid?