diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -624,6 +624,8 @@
         content = super().to_dict()
         if content["data"] is None:
             del content["data"]
+        if content["ctime"] is None:
+            del content["ctime"]
         return content
 
     @classmethod