diff --git a/radicale/storage.py b/radicale/storage.py
index d00c043..9728434 100644
--- a/radicale/storage.py
+++ b/radicale/storage.py
@@ -622,7 +622,7 @@ class BaseCollection:
         delta_props = self.get_meta()
         for key in delta_props.keys():
             if key not in props:
-                delta_props[key] = ""
+                delta_props[key] = None
         delta_props.update(props)
         self.set_meta(self, delta_props)