diff --git a/odoo/addons/test_new_api/tests/test_new_fields.py b/odoo/addons/test_new_api/tests/test_new_fields.py
index 9e5aabaaa38..2524a2e67f7 100644
--- a/odoo/addons/test_new_api/tests/test_new_fields.py
+++ b/odoo/addons/test_new_api/tests/test_new_fields.py
@@ -2878,6 +2878,21 @@ class TestX2many(common.TransactionCase):
         })
         self.assertTrue(field.unlink())
 
+    def test_custom_m2m_related(self):
+        # this checks the ondelete of a related many2many field
+        model_id = self.env['ir.model']._get_id('res.partner')
+        field = self.env['ir.model.fields'].create({
+            'name': 'x_foo',
+            'field_description': 'Foo',
+            'model_id': model_id,
+            'ttype': 'many2many',
+            'relation': 'res.partner.category',
+            'related': 'category_id',
+            'readonly': True,
+            'store': True,
+        })
+        self.assertTrue(field.unlink())
+
 
 class TestHtmlField(common.TransactionCase):
 
diff --git a/odoo/fields.py b/odoo/fields.py
index 256b5e37616..381034dc0f2 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -3720,7 +3720,7 @@ class Many2many(_RelationalMulti):
     column1 = None                      # column of table referring to model
     column2 = None                      # column of table referring to comodel
     auto_join = False                   # whether joins are generated upon search
-    ondelete = None                     # optional ondelete for the column2 fkey
+    ondelete = 'cascade'                # optional ondelete for the column2 fkey
 
     def __init__(self, comodel_name=Default, relation=Default, column1=Default,
                  column2=Default, string=Default, **kwargs):
@@ -3735,17 +3735,15 @@ class Many2many(_RelationalMulti):
 
     def setup_nonrelated(self, model):
         super().setup_nonrelated(model)
-        # 3 cases:
-        # 1) The ondelete attribute is not defined, we assign it a sensible default
-        # 2) The ondelete attribute is defined and its definition makes sense
-        # 3) The ondelete attribute is explicitly defined as 'set null' for a m2m,
+        # 2 cases:
+        # 1) The ondelete attribute is defined and its definition makes sense
+        # 2) The ondelete attribute is explicitly defined as 'set null' for a m2m,
         #    this is considered a programming error.
-        self.ondelete = self.ondelete or 'cascade'
-        if self.ondelete == 'set null':
+        if self.ondelete not in ('cascade', 'restrict'):
             raise ValueError(
                 "The m2m field %s of model %s declares its ondelete policy "
-                "as being 'set null'. Only 'restrict' and 'cascade' make sense."
-                % (self.name, model._name)
+                "as being %r. Only 'restrict' and 'cascade' make sense."
+                % (self.name, model._name, self.ondelete)
             )
         if self.store:
             if not (self.relation and self.column1 and self.column2):