diff --git a/src/calibre/ebooks/__init__.py b/src/calibre/ebooks/__init__.py
index d2d6fc94e1..a084cdd0f8 100644
--- a/src/calibre/ebooks/__init__.py
+++ b/src/calibre/ebooks/__init__.py
@@ -25,7 +25,7 @@ class DRMError(ValueError):
 BOOK_EXTENSIONS = ['lrf', 'rar', 'zip', 'rtf', 'lit', 'txt', 'htm', 'xhtm',
                    'html', 'xhtml', 'pdf', 'pdb', 'prc', 'mobi', 'azw', 'doc',
                    'epub', 'fb2', 'djvu', 'lrx', 'cbr', 'cbz', 'oebzip',
-                   'rb', 'imp', 'odt']
+                   'rb', 'imp', 'odt', 'chm']
 
 class HTMLRenderer(object):
 
diff --git a/src/calibre/ebooks/oeb/base.py b/src/calibre/ebooks/oeb/base.py
index e2a4875399..244acf7627 100644
--- a/src/calibre/ebooks/oeb/base.py
+++ b/src/calibre/ebooks/oeb/base.py
@@ -1327,7 +1327,9 @@ class TOC(object):
         return len(list(self.iter())) - 1
 
     def next_play_order(self):
-        return max([x.play_order for x in self.iter()])+1
+        entries = [x.play_order for x in self.iter()]
+        base = max(entries) if entries else 0
+        return base+1
 
     def has_href(self, href):
         for x in self.iter():