c = book.CustomerLookupByID(data['invoice']['invoicedata']['customerid'])
assert( isinstance(c, Customer) )
CUR = c.GetCurrency()
assert( isinstance(CUR, GncCommodity) )

invoice = Invoice(
             book=book,
             id=next_invoice_id(book=book, format=data['invoice']['invoicedata']['format_inv    oiceid']),
             currency=CUR,
             owner=c
             )

"""
PROBLEM:

this piece of code produces:

* 19:41:21  CRIT <gnc.backend.sql> gnc_sql_save_commodity: assertion `pCommodity != NULL' failed
* 19:41:21  WARN <gnc.backend.sql> [add_gvalue_owner_to_slist()] Invalid owner type: 0

both c and CUR are not None.
CUR is identical to book.get_table().lookup('CURRENCY', 'EUR').
c is a valid customer created with the normal gnucash GUI (.GetName() etc. are correct).

Any ideas?

"""