@@ -24,10 +24,11 @@
 /*!
  * @class OFNull OFNull.h ObjFW/OFNull.h
  *
  * @brief A class for representing null values in collections.
  */
+OF_SUBCLASSING_RESTRICTED
 @interface OFNull: OFObject <OFCopying, OFSerialization, OFJSONRepresentation,
     OFMessagePackRepresentation>
 /*!
  * @brief Returns an OFNull singleton.
  *