diff --git a/crans_utils.py b/crans_utils.py
index 0e5357d..3b358bc 100644
--- a/crans_utils.py
+++ b/crans_utils.py
@@ -350,10 +350,11 @@ def localized_datetime(date=None, tz=None):
     %Y%m%d%H%M%S, and a tz timezone looking like +0200"""
 
     if tz is None:
-        if "+" in date or '-' in date or 'Z' in date:
-            if date.endswith("Z"):
-                date = date[:-1] + "+0000"
-            date, tz = date[:-5], date[-5:]
+        if date is not None:
+            if "+" in date or '-' in date or 'Z' in date:
+                if date.endswith("Z"):
+                    date = date[:-1] + "+0000"
+                date, tz = date[:-5], date[-5:]
 
     _notz = (tz is None)