diff --git a/src/calibre/srv/render_book.py b/src/calibre/srv/render_book.py
index 87157fb30a..a1e12171ff 100644
--- a/src/calibre/srv/render_book.py
+++ b/src/calibre/srv/render_book.py
@@ -635,7 +635,7 @@ def virtualize_html(container, name, link_uid, link_to_map, virtualized_names):
     return name in changed
 
 
-__smil_file_names__ = None
+__smil_file_names__ = ''
 
 
 def process_book_files(names, container_dir, opfpath, virtualize_resources, link_uid, data_for_clone, container=None):