diff --git a/docs/ref/models/fields.txt b/docs/ref/models/fields.txt
index 869996643f..7159ed8b70 100644
--- a/docs/ref/models/fields.txt
+++ b/docs/ref/models/fields.txt
@@ -874,6 +874,9 @@ are converted to lowercase.
     ``192.0.2.1``. Default is disabled. Can only be used
     when ``protocol`` is set to ``'both'``.
 
+If you allow for blank values, you have to allow for null values since blank
+values are stored as null.
+
 ``NullBooleanField``
 --------------------