diff --git a/account_banking_mandate/models/account_move_line.py b/account_banking_mandate/models/account_move_line.py
index 3df33cafe..b011c817e 100644
--- a/account_banking_mandate/models/account_move_line.py
+++ b/account_banking_mandate/models/account_move_line.py
@@ -1,5 +1,5 @@
 # Copyright Akretion (http://www.akretion.com/)
-# Copyright 2017 Carlos Dauden <carlos.dauden@tecnativa.com>
+# Copyright 2017-2021 Carlos Dauden <carlos.dauden@tecnativa.com>
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
 
 
@@ -25,12 +25,8 @@ class AccountMoveLine(models.Model):
             mandate = mandate.browse(vals['mandate_id'])
         partner_bank_id = vals.get('partner_bank_id', False)
         if not mandate:
-            if partner_bank_id:
-                domain = [('partner_bank_id', '=', partner_bank_id)]
-            else:
-                domain = [('partner_id', '=', self.partner_id.id)]
-            domain.append(('state', '=', 'valid'))
-            mandate = mandate.search(domain, limit=1)
+            mandate = self.partner_id._get_first_valid_mandate(
+                partner_bank_id=partner_bank_id)
         vals.update({
             'mandate_id': mandate.id,
             'partner_bank_id': mandate.partner_bank_id.id or partner_bank_id,
diff --git a/account_banking_mandate/models/res_partner.py b/account_banking_mandate/models/res_partner.py
index 8bfa022a0..39163d843 100644
--- a/account_banking_mandate/models/res_partner.py
+++ b/account_banking_mandate/models/res_partner.py
@@ -1,8 +1,9 @@
 # Copyright 2016 Akretion (Alexis de Lattre <alexis.delattre@akretion.com>)
-# Copyright 2017 Carlos Dauden <carlos.dauden@tecnativa.com>
+# Copyright 2017-2021 Carlos Dauden <carlos.dauden@tecnativa.com>
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
 
 from odoo import models, fields, api
+from odoo.fields import first
 
 
 class ResPartner(models.Model):
@@ -26,25 +27,31 @@ class ResPartner(models.Model):
         for partner in self:
             partner.mandate_count = mapped_data.get(partner.id, 0)
 
-    @api.multi
-    def _compute_valid_mandate_id(self):
-        # Dict for reducing the duplicated searches on parent/child partners
+    def _get_first_valid_mandate(self, partner_bank_id=False):
+        self.ensure_one()
         company_id = self.env.context.get('force_company', False)
         if company_id:
             company = self.env['res.company'].browse(company_id)
         else:
             company = self.env['res.company']._company_default_get(
                 'account.banking.mandate')
+        mandates = self.commercial_partner_id.bank_ids.mapped('mandate_ids')
+        mandates = mandates.filtered(
+            lambda x: x.state == 'valid' and x.company_id == company)
+        if partner_bank_id:
+            mandates = mandates.filtered(
+                lambda m: m.partner_bank_id.id == partner_bank_id) or mandates
+        return first(mandates)
 
+    @api.multi
+    def _compute_valid_mandate_id(self):
+        # Dict for reducing the duplicated searches on parent/child partners
         mandates_dic = {}
         for partner in self:
             commercial_partner_id = partner.commercial_partner_id.id
             if commercial_partner_id in mandates_dic:
                 partner.valid_mandate_id = mandates_dic[commercial_partner_id]
             else:
-                mandates = partner.commercial_partner_id.bank_ids.mapped(
-                    'mandate_ids').filtered(
-                    lambda x: x.state == 'valid' and x.company_id == company)
-                first_valid_mandate_id = mandates[:1].id
+                first_valid_mandate_id = partner._get_first_valid_mandate()
                 partner.valid_mandate_id = first_valid_mandate_id
                 mandates_dic[commercial_partner_id] = first_valid_mandate_id
diff --git a/account_banking_mandate/tests/test_invoice_mandate.py b/account_banking_mandate/tests/test_invoice_mandate.py
index 5ccf5bcd3..51f0f60f2 100644
--- a/account_banking_mandate/tests/test_invoice_mandate.py
+++ b/account_banking_mandate/tests/test_invoice_mandate.py
@@ -60,6 +60,25 @@ class TestInvoiceMandate(TransactionCase):
             with self.assertRaises(ValidationError):
                 payable_move_lines[0].mandate_id = mandate_2
 
+    def test_post_invoice_default_mandate(self):
+        self.invoice.mandate_id = False
+        self.invoice.partner_bank_id = self.mandate.partner_bank_id
+
+        self.invoice.action_invoice_open()
+
+        self.env['account.invoice.payment.line.multi'].with_context(
+            active_model='account.invoice',
+            active_ids=self.invoice.ids
+        ).create({}).run()
+
+        payment_order = self.env['account.payment.order'].search([])
+        self.assertEqual(len(payment_order.ids), 1)
+        payment_order.payment_mode_id_change()
+        payment_order.draft2open()
+        payment_order.open2generated()
+        payment_order.generated2uploaded()
+        self.assertEqual(self.mandate.payment_line_ids_count, 1)
+
     def test_post_invoice_and_refund_02(self):
         self.invoice._onchange_partner_id()
         self.invoice.action_invoice_open()