diff --git a/converters/python.py b/converters/python.py
index 5b0bf2d..7607e05 100755
--- a/converters/python.py
+++ b/converters/python.py
@@ -33,16 +33,21 @@ class ConverterPy(Converter):
         return ['#{0} {1}'.format('#'*cell.level, cell.source), '']
 
     def render_code(self, cell):
+        try:
+            prompt_number = cell.prompt_number
+        except AttributeError:
+            prompt_number = " "
+
         if not cell.input:
             return []
         lines = []
         if self.show_prompts:
-            lines.extend(['# In[%s]:' % cell.prompt_number])
+            lines.extend(['# In[%s]:' % prompt_number])
         src = cell.input
         lines.extend([src, ''])
         if self.show_output:
             if cell.outputs :
-                lines.extend(['# Out[%s]:' % cell.prompt_number])
+                lines.extend(['# Out[%s]:' % prompt_number])
             for output in cell.outputs:
                 conv_fn = self.dispatch(output.output_type)
                 lines.extend(conv_fn(output))
diff --git a/converters/rst.py b/converters/rst.py
index 1e626ee..09ddcaf 100755
--- a/converters/rst.py
+++ b/converters/rst.py
@@ -11,10 +11,16 @@ class ConverterRST(Converter):
         return ['{0}\n{1}\n'.format(cell.source, marker * len(cell.source))]
 
     def render_code(self, cell):
+        # Note: cell has type 'IPython.nbformat.v3.nbbase.NotebookNode'
         if not cell.input:
             return []
 
-        lines = ['In[%s]:' % cell.prompt_number, '']
+        try:
+            prompt_number = cell.prompt_number
+        except AttributeError:
+            prompt_number = " "
+
+        lines = ['In[%s]:' % prompt_number, '']
         lines.extend(rst_directive('.. code:: python', cell.input))
 
         for output in cell.outputs: