diff --git a/api-openbis-python3-pybis/src/python/pybis/property_reformatter.py b/api-openbis-python3-pybis/src/python/pybis/property_reformatter.py
index 12bab0878747b87643d05cb0bdcd4f0217cc7904..59739cd644f6dd70d738057f362c9a4fc6a3f46b 100644
--- a/api-openbis-python3-pybis/src/python/pybis/property_reformatter.py
+++ b/api-openbis-python3-pybis/src/python/pybis/property_reformatter.py
@@ -51,6 +51,8 @@ class PropertyReformatter:
         return properties
 
     def _format_timestamp(self, value):
+        if value is None:
+            return value
         if is_of_openbis_supported_date_format(value):
             return value
         timestamp = pd.to_datetime(value)