--- a/cubicweb/web/formfields.py	Thu Jan 26 16:33:15 2017 +0100
+++ b/cubicweb/web/formfields.py	Thu Jan 26 16:34:56 2017 +0100
@@ -1027,6 +1027,8 @@
     def _ensure_correctly_typed(self, form, value):
         tz_naive = super(TZDatetimeField, self)._ensure_correctly_typed(
             form, value)
+        if not tz_naive:
+            return None
         return tz_naive.replace(tzinfo=pytz.utc)