diff --git a/src/calibre/__init__.py b/src/calibre/__init__.py
index 347799de64..6faa021151 100644
--- a/src/calibre/__init__.py
+++ b/src/calibre/__init__.py
@@ -92,6 +92,7 @@ def sanitize_file_name(name, substitute='_', as_unicode=False):
     if as_unicode:
         one = one.decode(filesystem_encoding)
     one = one.replace('..', substitute)
+    # Windows doesn't like path components that end with a period
     if one.endswith('.'):
         one = one[:-1]+'_'
     return one