diff --git a/HACKING.rst b/HACKING.rst
index c64d0d752d..6376b317a0 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -23,3 +23,4 @@ Magnum Specific Commandments
   assertIn/NotIn(A, B, message)
 - [M336] Must use a dict comprehension instead of a dict constructor
   with a sequence of key-value pairs.
+- [M338] Use assertIn/NotIn(A, B) rather than assertEqual(A in B, True/False).