diff --git a/IPython/nbconvert/preprocessors/clearoutput.py b/IPython/nbconvert/preprocessors/clearoutput.py
index 61bad66..61c4b7f 100644
--- a/IPython/nbconvert/preprocessors/clearoutput.py
+++ b/IPython/nbconvert/preprocessors/clearoutput.py
@@ -24,5 +24,5 @@ class ClearOutputPreprocessor(Preprocessor):
         """
         if cell.cell_type == 'code':
             cell.outputs = []
-            cell['prompt_number'] = None
+            cell.prompt_number = None
         return cell, resources