diff --git a/vdirsyncer/storage/http.py b/vdirsyncer/storage/http.py
index f932b59..978b7de 100644
--- a/vdirsyncer/storage/http.py
+++ b/vdirsyncer/storage/http.py
@@ -48,6 +48,12 @@ class HttpStorage(Storage):
     Use a simple ``.ics`` file (or similar) from the web. Usable as ``http`` in
     the config file.
 
+    .. note::
+
+        This is a read-only storage. If you sync this with
+        read-and-write-storages (such as CalDAV), make sure not to change
+        anything on the other side, otherwise vdirsyncer will crash.
+
     :param url: URL to the ``.ics`` file.
     :param username: Username for authentication.
     :param password: Password for authentication.