diff --git a/src/globals.h b/src/globals.h
index 1fadc74..21fabdb 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1035,7 +1035,8 @@
  * (DBCS).
  * The value is set in mb_init();
  */
-// length of char in bytes, including following composing chars
+// Length of char in bytes, including any following composing chars.
+// NUL has length zero.
 EXTERN int (*mb_ptr2len)(char_u *p) INIT(= latin_ptr2len);
 
 // idem, with limit on string length