diff --git a/changelog.d/7035.bugfix b/changelog.d/7035.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..56292dc8ac2f08f0b8328170d01975c475024082
--- /dev/null
+++ b/changelog.d/7035.bugfix
@@ -0,0 +1 @@
+Fix a bug causing `org.matrix.dummy_event` to be included in responses from `/sync`.
diff --git a/synapse/visibility.py b/synapse/visibility.py
index e60d9756b78f3cde0a1d916d570762b67f48898d..a48a4f3dfe495f2d778221a84a3c1966e600307f 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -119,6 +119,9 @@ def filter_events_for_client(
 
                the original event if they can see it as normal.
         """
+        if event.type == "org.matrix.dummy_event":
+            return None
+
         if not event.is_state() and event.sender in ignore_list:
             return None