diff --git a/changelog.d/8628.bugfix b/changelog.d/8628.bugfix
new file mode 100644
index 000000000..1316136ca
--- /dev/null
+++ b/changelog.d/8628.bugfix
@@ -0,0 +1 @@
+Fix handling of invalid group IDs to return a 400 rather than log an exception and return a 500.
diff --git a/synapse/handlers/groups_local.py b/synapse/handlers/groups_local.py
index 9684e60fc..b2def93bb 100644
--- a/synapse/handlers/groups_local.py
+++ b/synapse/handlers/groups_local.py
@@ -17,7 +17,7 @@
 import logging
 
 from synapse.api.errors import HttpResponseException, RequestSendFailed, SynapseError
-from synapse.types import get_domain_from_id
+from synapse.types import GroupID, get_domain_from_id
 
 logger = logging.getLogger(__name__)
 
@@ -28,6 +28,9 @@ def _create_rerouter(func_name):
     """
 
     async def f(self, group_id, *args, **kwargs):
+        if not GroupID.is_valid(group_id):
+            raise SynapseError(400, "%s was not legal group ID" % (group_id,))
+
         if self.is_mine_id(group_id):
             return await getattr(self.groups_server_handler, func_name)(
                 group_id, *args, **kwargs