diff --git a/nbrmd/contentsmanager.py b/nbrmd/contentsmanager.py
index 6bfe57d59..fc613fff7 100644
--- a/nbrmd/contentsmanager.py
+++ b/nbrmd/contentsmanager.py
@@ -112,7 +112,7 @@ def all_nb_extensions(self):
         return ['.ipynb'] + self.nb_extensions
 
     default_nbrmd_formats = Unicode(
-        u'ipynb',
+        u'',
         help='Save notebooks to these file extensions. '
              'Can be any of ipynb,Rmd,py,R,nb.py,nb.R comma separated',
         config=True)