diff --git a/docs/db-api.txt b/docs/db-api.txt
index ce6bb0ab3b..a7cf30813d 100644
--- a/docs/db-api.txt
+++ b/docs/db-api.txt
@@ -578,6 +578,9 @@ related ``Person`` *and* the related ``City``::
     p = b.author         # Hits the database.
     c = p.hometown       # Hits the database.
 
+Note that ``select_related()`` does not follow foreign keys that have
+``null=True``.
+
 ``extra(select=None, where=None, params=None, tables=None)``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~