diff --git a/odoo/fields.py b/odoo/fields.py
index 854a2f831b2..8b7cbfe71cc 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -836,7 +836,7 @@ class Field(MetaField('DummyField', (object,), {})):
             def add_not_null():
                 # flush values before adding NOT NULL constraint
                 model.flush([self.name])
-                model.pool.post_constraint(sql.set_not_null, model._cr, model._table, self.name)
+                model.pool.post_constraint(apply_required, model, self.name)
 
         elif not self.required and has_notnull:
             sql.drop_not_null(model._cr, model._table, self.name)
@@ -3702,6 +3702,16 @@ def prefetch_x2many_ids(record, field):
     return unique(id_ for ids in ids_list for id_ in ids)
 
 
+def apply_required(model, field_name):
+    """ Set a NOT NULL constraint on the given field, if necessary. """
+    # At the time this function is called, the model's _fields may have been reset, although
+    # the model's class is still the same. Retrieve the field to see whether the NOT NULL
+    # constraint still applies
+    field = model._fields[field_name]
+    if field.store and field.required:
+        sql.set_not_null(model.env.cr, model._table, field_name)
+
+
 # imported here to avoid dependency cycle issues
 from .exceptions import AccessError, MissingError, UserError
 from .models import check_pg_name, BaseModel, NewId, IdType