diff --git a/src/calibre/utils/fonts/sfnt/container.py b/src/calibre/utils/fonts/sfnt/container.py
index 73bb787810..2e08ef3cee 100644
--- a/src/calibre/utils/fonts/sfnt/container.py
+++ b/src/calibre/utils/fonts/sfnt/container.py
@@ -22,6 +22,8 @@ from calibre.utils.fonts.sfnt.loca import LocaTable
 from calibre.utils.fonts.sfnt.glyf import GlyfTable
 from calibre.utils.fonts.sfnt.cmap import CmapTable
 
+# OpenType spec: http://www.microsoft.com/typography/otspec/otff.htm
+
 class Sfnt(object):
 
     def __init__(self, raw):