diff --git a/radicale/storage.py b/radicale/storage.py
index fec418b..e11d67b 100644
--- a/radicale/storage.py
+++ b/radicale/storage.py
@@ -262,12 +262,6 @@ class BaseCollection:
         """
         raise NotImplementedError
 
-    @contextmanager
-    def at_once(self):
-        """Set a context manager buffering the reads and writes."""
-        # TODO: use in code
-        yield
-
     def get_meta(self, key):
         """Get metadata value for collection."""
         raise NotImplementedError
@@ -474,11 +468,6 @@ class Collection(BaseCollection):
                 "Can't tranlate name safely to filesystem, "
                 "skipping component: %s", href)
 
-    @contextmanager
-    def at_once(self):
-        # TODO: use a file locker
-        yield
-
     def get_meta(self, key):
         props_path = self._filesystem_path + ".props"
         if os.path.exists(props_path):
diff --git a/radicale/xmlutils.py b/radicale/xmlutils.py
index 8ce63c6..12d0349 100644
--- a/radicale/xmlutils.py
+++ b/radicale/xmlutils.py
@@ -438,7 +438,11 @@ def _propfind_response(path, item, props, user, write=False):
                 element.append(supported)
         elif is_collection:
             if tag == _tag("D", "getcontenttype"):
-                element.text = storage.MIMETYPES[item.get_meta("tag")]
+                item_tag = item.get_meta("tag")
+                if item_tag:
+                    element.text = storage.MIMETYPES[item_tag]
+                else:
+                    is404 = True
             elif tag == _tag("D", "resourcetype"):
                 if item.is_principal:
                     tag = ET.Element(_tag("D", "principal"))