diff --git a/gluon/dal.py b/gluon/dal.py
index 73783a45..1aa424aa 100644
--- a/gluon/dal.py
+++ b/gluon/dal.py
@@ -6876,8 +6876,8 @@ class Row(object):
         key=str(k)
         _extra = self.__dict__.get('_extra', None)
         if _extra is not None:
-            v = _extra.get(key, None)
-            if v is not None:
+            v = _extra.get(key, r'[__+-+_key_not_found_+-+__]')
+            if v != r'[__+-+_key_not_found_+-+__]':
                 return v
         m = REGEX_TABLE_DOT_FIELD.match(key)
         if m:
diff --git a/gluon/tests/test_dal.py b/gluon/tests/test_dal.py
index ab444958..8a83282a 100644
--- a/gluon/tests/test_dal.py
+++ b/gluon/tests/test_dal.py
@@ -387,6 +387,10 @@ class TestExpressions(unittest.TestCase):
         self.assertEqual(db.tt.insert(aa=3), 3)
         self.assertEqual(db(db.tt.aa == 3).update(aa=db.tt.aa + 1), 1)
         self.assertEqual(db(db.tt.aa == 4).count(), 1)
+        self.assertEqual(db(db.tt.aa == -2).count(), 0)
+        sum = (db.tt.aa + 1).sum()
+        self.assertEqual(db(db.tt.aa == 2).select(sum).first()[sum], 3)
+        self.assertEqual(db(db.tt.aa == -2).select(sum).first()[sum], None)
         db.tt.drop()