diff --git a/lib/zip.h b/lib/zip.h
index f605bcf..4a1bf89 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -163,6 +163,7 @@
 /* 15-17 - Reserved by PKWARE */
 #define ZIP_CM_TERSE 18 /* compressed using IBM TERSE (new) */
 #define ZIP_CM_LZ77 19  /* IBM LZ77 z Architecture (PFS) */
+/* 20 - old value for Zstandard */
 #define ZIP_CM_LZMA2 33
 #define ZIP_CM_ZSTD 93    /* Zstandard compressed data */
 #define ZIP_CM_XZ 95      /* XZ compressed data */