Hello, 

This is my function: 

@api.depends('xidentification')
    def _concat_nit(self):

        # Executing only for Document Type 31 (NIT)
        if self.doctype is 31: # <--------------- THIS IS LINE 147
            # First check if entered value is valid
            self._check_ident()
            self._check_ident_num()
            # Instead of showing "False" we put en empty string
            if self.xidentification is False:
                self.xidentification = ''
            self.formatedNit = ''
            # Formatting the NIT: xx.xxx.xxx-x
            s = str(self.xidentification)[::-1]
            newnit = '.'.join(s[i:i+3] for i in range(0, len(s), 3))
            newnit = newnit[::-1]
            nitList = [
                newnit,
                # Calling the NIT Function which creates the Verification Code:
                self._check_dv(str(self.xidentification))
            ]
            formatedNitList = []
            for item in nitList:
                if item is not '':
                    formatedNitList.append(item)
                    self.formatedNit = '-' .join(formatedNitList)
            # Saving Verification digit in a proper field
            self.dv = nitList[1]

But I get the following error: 

File "/home/travis/build/OCA/l10n-colombia/l10n_co_res_partner/models/l10n_co_res_partner.py", line 147, in _concat_nit
if self.doctype is 31:
File "/home/travis/odoo-9.0/openerp/fields.py", line 819, in __get__
record.ensure_one()
File "/home/travis/odoo-9.0/openerp/models.py", line 5403, in ensure_one
raise ValueError("Expected singleton: %s" % self)

ValueError: Expected singleton: res.partner(3, 7, 17, 32, 22, 21, 6, 18, 16, 14, 19, 20, 13, 11, 29, 35, 28, 8, 23, 24, 34, 25, 9, 26, 27, 39, 38, 15, 33, 37, 43, 5, 4, 42, 10, 30, 31, 12, 36, 1, 40, 41)

I have no clue why I get this error? Actually the function works in the interface, but Travis is complaining: https://travis-ci.org/OCA/l10n-colombia/jobs/139356438#L645

Any Ideas on how to fix this issue?