diff --git a/IPython/nbconvert/preprocessors/clearoutput.py b/IPython/nbconvert/preprocessors/clearoutput.py
index 519a0af..61bad66 100644
--- a/IPython/nbconvert/preprocessors/clearoutput.py
+++ b/IPython/nbconvert/preprocessors/clearoutput.py
@@ -24,6 +24,5 @@ class ClearOutputPreprocessor(Preprocessor):
         """
         if cell.cell_type == 'code':
             cell.outputs = []
-            if 'prompt_number' in cell:
-                del cell['prompt_number']
+            cell['prompt_number'] = None
         return cell, resources
diff --git a/IPython/nbconvert/preprocessors/tests/test_clearoutput.py b/IPython/nbconvert/preprocessors/tests/test_clearoutput.py
index 1b0986d..9434f7c 100644
--- a/IPython/nbconvert/preprocessors/tests/test_clearoutput.py
+++ b/IPython/nbconvert/preprocessors/tests/test_clearoutput.py
@@ -39,4 +39,4 @@ class TestClearOutput(PreprocessorTestsBase):
         preprocessor = self.build_preprocessor()
         nb, res = preprocessor(nb, res)
         assert nb.worksheets[0].cells[0].outputs == []
-        assert 'prompt_number' not in nb.worksheets[0].cells[0]
+        assert nb.worksheets[0].cells[0].prompt_number is None