diff --git a/changelog.d/11930.bugfix b/changelog.d/11930.bugfix
new file mode 100644
index 0000000000..e0dfbf1a15
--- /dev/null
+++ b/changelog.d/11930.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug that some unknown endpoints would return HTML error pages instead of JSON `M_UNRECOGNIZED` errors.
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index 24d55b0494..66e1a21331 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -26,6 +26,7 @@ import synapse
 import synapse.config.logger
 from synapse import events
 from synapse.api.urls import (
+    CLIENT_API_PREFIX,
     FEDERATION_PREFIX,
     LEGACY_MEDIA_PREFIX,
     MEDIA_R0_PREFIX,
@@ -195,13 +196,7 @@ class SynapseHomeServer(HomeServer):
 
             resources.update(
                 {
-                    "/_matrix/client/api/v1": client_resource,
-                    "/_matrix/client/r0": client_resource,
-                    "/_matrix/client/v1": client_resource,
-                    "/_matrix/client/v3": client_resource,
-                    "/_matrix/client/unstable": client_resource,
-                    "/_matrix/client/v2_alpha": client_resource,
-                    "/_matrix/client/versions": client_resource,
+                    CLIENT_API_PREFIX: client_resource,
                     "/.well-known": well_known_resource(self),
                     "/_synapse/admin": AdminRestResource(self),
                     **build_synapse_client_resource_tree(self),