diff --git a/doc/coding-style.txt b/doc/coding-style.txt
index 6fa355ed..fb43891f 100644
--- a/doc/coding-style.txt
+++ b/doc/coding-style.txt
@@ -205,6 +205,10 @@ default:  // wrong
 	break;
 }
 
+However if the enum comes from an external header file outside ofono
+we cannot make any assumption of how the enum is defined and this
+rule might not apply.
+
 O1: Shorten the name
 ====================
 Better to use abbreviation, rather than full name, to name a variable,