diff --git a/tailbone/views/reports.py b/tailbone/views/reports.py
index c9a14be7..b9850ed6 100644
--- a/tailbone/views/reports.py
+++ b/tailbone/views/reports.py
@@ -415,7 +415,7 @@ class GenerateReport(View):
 
             # allow empty value if param is optional
             if not param.required:
-                node.missing = colander.null
+                node.missing = None
 
             # maybe set default value
             if hasattr(param, 'default'):