append() with datatable in content object

Hello,

I'd like to insert a new entry into a data table from within a python  
script:

...
customers   = context.content.e4.e41.data_customers
rec_c       = customers.data_customers.getObjProperty('records',  
request)
id          = zms.getNewId()

rec_c.append({'id':  
id ,'col_lastname':'Taenzer','col_firstname':'Sebastian'})

data_customers is the data table inside the special object.

This does not work.

If I use this script with a datatable that has an id (i.e. "e35.e77")  
it works fine.

Any ideas what's the reason for this?

Thanks,
Sebastian