diff --git a/IPython/nbformat/v3/v3.withref.json b/IPython/nbformat/v3/v3.withref.json
index cc0031a..8839f89 100644
--- a/IPython/nbformat/v3/v3.withref.json
+++ b/IPython/nbformat/v3/v3.withref.json
@@ -153,7 +153,7 @@
             "type": "array"
         },
         "prompt_number": {
-            "type": ["number","string"],
+            "type": ["number","null"],
             "minimum": 0
         },
         "language": {