diff --git a/modules/restricted-guests/synapse/synapse_guest_module/guest_module.py b/modules/restricted-guests/synapse/synapse_guest_module/guest_module.py
index 281cf9dd6b..7f53f5052e 100644
--- a/modules/restricted-guests/synapse/synapse_guest_module/guest_module.py
+++ b/modules/restricted-guests/synapse/synapse_guest_module/guest_module.py
@@ -218,6 +218,7 @@ class GuestModule:
             """
             CREATE TABLE IF NOT EXISTS guest_module_mas_users (
                 mas_user_id TEXT PRIMARY KEY,
+                user_id TEXT,
                 created_at BIGINT NOT NULL
             )
             """,
diff --git a/modules/restricted-guests/synapse/synapse_guest_module/guest_registration_servlet.py b/modules/restricted-guests/synapse/synapse_guest_module/guest_registration_servlet.py
index fcf1469161..41257e5726 100644
--- a/modules/restricted-guests/synapse/synapse_guest_module/guest_registration_servlet.py
+++ b/modules/restricted-guests/synapse/synapse_guest_module/guest_registration_servlet.py
@@ -104,7 +104,7 @@ class GuestRegistrationServlet(DirectServeJsonResource):
                     displayname + self._config.display_name_suffix,
                 )
 
-                await self._store_mas_user(mas_user_id, int(time.time()))
+                await self._store_mas_user(mas_user_id, user_id, int(time.time()))
 
                 # Determine how long to keep the access token valid for.
                 #
@@ -135,7 +135,7 @@ class GuestRegistrationServlet(DirectServeJsonResource):
 
         return 500, {"msg": "Internal error: Could not find a free username"}
 
-    async def _store_mas_user(self, mas_user_id: str, created_at: int) -> None:
+    async def _store_mas_user(self, mas_user_id: str, user_id: str, created_at: int) -> None:
         if self._mas_tables_ready is not None:
             await self._mas_tables_ready.wait()
 
@@ -145,6 +145,7 @@ class GuestRegistrationServlet(DirectServeJsonResource):
                 table="guest_module_mas_users",
                 values={
                     "mas_user_id": mas_user_id,
+                    "user_id": user_id,
                     "created_at": created_at,
                 },
             )
diff --git a/modules/restricted-guests/synapse/tests/__init__.py b/modules/restricted-guests/synapse/tests/__init__.py
index d79a60a891..b97dd1c45b 100644
--- a/modules/restricted-guests/synapse/tests/__init__.py
+++ b/modules/restricted-guests/synapse/tests/__init__.py
@@ -160,5 +160,5 @@ def _setup_db(conn: sqlite3.Connection) -> None:
         "CREATE TABLE users(name text, deactivated smallint, creation_ts bigint)"
     )
     conn.execute(
-        "CREATE TABLE guest_module_mas_users(mas_user_id text, created_at bigint)"
+        "CREATE TABLE guest_module_mas_users(mas_user_id text, user_id text, created_at bigint)"
     )
diff --git a/modules/restricted-guests/synapse/tests/test_guest_user_reaper.py b/modules/restricted-guests/synapse/tests/test_guest_user_reaper.py
index 06815e1427..960e3f999f 100644
--- a/modules/restricted-guests/synapse/tests/test_guest_user_reaper.py
+++ b/modules/restricted-guests/synapse/tests/test_guest_user_reaper.py
@@ -136,10 +136,10 @@ class GuestUserReaperTest(aiounittest.AsyncTestCase):
 
         now = int(time.time())
         store.conn.executemany(
-            "INSERT INTO guest_module_mas_users VALUES (?, ?)",
+            "INSERT INTO guest_module_mas_users VALUES (?, ?, ?)",
             [
-                ["mas-old-1", 0],
-                ["mas-active", now],
+                ["mas-old-1", "@old-1:localhost", 0],
+                ["mas-active", "@active:localhost", now],
             ],
         )
 
@@ -161,6 +161,6 @@ class GuestUserReaperTest(aiounittest.AsyncTestCase):
         )
 
         remaining_users = store.conn.execute(
-            "SELECT mas_user_id FROM guest_module_mas_users"
+            "SELECT mas_user_id, user_id FROM guest_module_mas_users"
         ).fetchall()
-        self.assertEqual(remaining_users, [("mas-active",)])
+        self.assertEqual(remaining_users, [("mas-active", "@active:localhost")])