diff --git a/en/reference/architecture.rst b/en/reference/architecture.rst
index 9a1d5ee44..c3508bf7b 100644
--- a/en/reference/architecture.rst
+++ b/en/reference/architecture.rst
@@ -15,7 +15,8 @@ be any regular PHP class observing the following restrictions:
 -  An entity class must not be final or contain final methods.
 -  All persistent properties/field of any entity class should
    always be private or protected, otherwise lazy-loading might not
-   work as expected.
+   work as expected. In case you serialize entities (for example Session)
+   properties should be protected (See Serialize section below).
 -  An entity class must not implement ``__clone`` or
    :doc:`do so safely <../cookbook/implementing-wakeup-or-clone>`.
 -  An entity class must not implement ``__wakeup`` or