diff --git a/aiida/common/timezone.py b/aiida/common/timezone.py
index f940b9f17f..0727e2c68c 100644
--- a/aiida/common/timezone.py
+++ b/aiida/common/timezone.py
@@ -124,6 +124,8 @@ def datetime_to_isoformat(value):
 
     :param value: a datetime object
     """
+    if value is None:
+        return None
     return value.isoformat()
 
 
@@ -132,4 +134,6 @@ def isoformat_to_datetime(value):
 
     :param value: a ISO format string representation of a datetime object
     """
+    if value is None:
+        return None
     return dateutil.parser.parse(value)