From 1c91a954e84f25d9f488180f3612a8bf1f05e343 2013-05-21 07:32:42
From: Jonathan Frederic <jdfreder@calpoly.edu>
Date: 2013-05-21 07:32:42
Subject: [PATCH] Fixed None transformers error.

---

diff --git a/nbconvert/api/exporter.py b/nbconvert/api/exporter.py
index 520d505..6aa3505 100755
--- a/nbconvert/api/exporter.py
+++ b/nbconvert/api/exporter.py
@@ -196,6 +196,8 @@ class Exporter(Configurable):
         ----------
         transformer : transformer
         """
+        if self.transformers is None:
+            self.transformers = []
         
         if inspect.isfunction(transformer):
             self.transformers.append(transformer)