diff --git a/addons/base_vat/tests/test_validate_ruc.py b/addons/base_vat/tests/test_validate_ruc.py
index f542e794887..d7f3eccbbb7 100644
--- a/addons/base_vat/tests/test_validate_ruc.py
+++ b/addons/base_vat/tests/test_validate_ruc.py
@@ -59,6 +59,32 @@ class TestStructure(TransactionCase):
                 company.vat = "BE0987654321"  # VIES refused, don't fallback on other check
             company.vat = "BE0477472701"
 
+    def test_vat_syntactic_validation(self):
+        """ Tests VAT validation (both successes and failures), with the different country
+        detection cases possible.
+        """
+        test_partner = self.env['res.partner'].create({'name': "John Dex"})
+
+        # VAT starting with country code: use the starting country code
+        test_partner.write({'vat': 'BE0477472701', 'country_id': self.env.ref('base.fr').id})
+        test_partner.write({'vat': 'BE0477472701', 'country_id': None})
+
+        with self.assertRaises(ValidationError):
+            test_partner.write({'vat': 'BE42', 'country_id': self.env.ref('base.fr').id})
+
+        with self.assertRaises(ValidationError):
+            test_partner.write({'vat': 'BE42', 'country_id': None})
+
+        # No country code in VAT: use the partner's country
+        test_partner.write({'vat': '0477472701', 'country_id': self.env.ref('base.be').id})
+
+        with self.assertRaises(ValidationError):
+            test_partner.write({'vat': '42', 'country_id': self.env.ref('base.be').id})
+
+        # If no country can be guessed: VAT number cannot be validated
+        with self.assertRaises(ValidationError):
+            test_partner.write({'vat': '0477472701', 'country_id': None})
+
 
 @tagged('-standard', 'external')
 class TestStructureVIES(TestStructure):
diff --git a/odoo/addons/base/tests/test_base.py b/odoo/addons/base/tests/test_base.py
index 15d9108c08f..98ad34afb51 100644
--- a/odoo/addons/base/tests/test_base.py
+++ b/odoo/addons/base/tests/test_base.py
@@ -383,12 +383,12 @@ class TestBase(TransactionCase):
         for p in (p0, p1, p11, p2, p3):
             self.assertEqual(p.commercial_partner_id, sunhelm, 'Incorrect commercial entity resolution')
             self.assertEqual(p.vat, sunhelm.vat, 'Commercial fields must be automatically synced')
-        sunhelmvat = 'BE0123456789'
+        sunhelmvat = 'BE0123456749'
         sunhelm.write({'vat': sunhelmvat})
         for p in (p0, p1, p11, p2, p3):
             self.assertEqual(p.vat, sunhelmvat, 'Commercial fields must be automatically and recursively synced')
 
-        p1vat = 'BE0987654321'
+        p1vat = 'BE0987654394'
         p1.write({'vat': p1vat})
         for p in (sunhelm, p0, p11, p2, p3):
             self.assertEqual(p.vat, sunhelmvat, 'Sync to children should only work downstream and on commercial entities')
@@ -401,7 +401,7 @@ class TestBase(TransactionCase):
         self.assertEqual(p1.commercial_partner_id, p1, 'Incorrect commercial entity resolution after setting is_company')
 
         # writing on parent should not touch child commercial entities
-        sunhelmvat2 = 'BE0112233445'
+        sunhelmvat2 = 'BE0112233453'
         sunhelm.write({'vat': sunhelmvat2})
         self.assertEqual(p1.vat, p1vat, 'Setting is_company should stop auto-sync of commercial fields')
         self.assertEqual(p0.vat, sunhelmvat2, 'Commercial fields must be automatically synced')