diff --git a/gluon/dal/adapters/base.py b/gluon/dal/adapters/base.py
index f672b0f7..29aacd52 100644
--- a/gluon/dal/adapters/base.py
+++ b/gluon/dal/adapters/base.py
@@ -1323,7 +1323,7 @@ class BaseAdapter(ConnectionPool):
             value = fieldtype.encoder(obj)
             if fieldtype.type in ('string','text', 'json'):
                 return self.adapt(value)
-            return value
+            return value or 'NULL'
         if isinstance(obj, (Expression, Field)):
             return str(obj)
         if field_is_type('list:'):