diff --git a/nbconvert.py b/nbconvert.py
index f4ea5bd..ff08f6b 100755
--- a/nbconvert.py
+++ b/nbconvert.py
@@ -90,11 +90,11 @@ class Converter(object):
             f.write(self.output.encode(encoding))
         return infile
 
-    def optional_header():
-        pass
+    def optional_header(self):
+        return []
 
-    def optional_footer():
-        pass
+    def optional_footer(self):
+        return []
 
     def render_heading(self, cell):
         """convert a heading cell