diff --git a/plugins/auth-backend/src/service/router.ts b/plugins/auth-backend/src/service/router.ts
index 9841df399a..e988fde97c 100644
--- a/plugins/auth-backend/src/service/router.ts
+++ b/plugins/auth-backend/src/service/router.ts
@@ -144,6 +144,11 @@ export async function createRouter({
     }
   }
 
+  router.use('/:provider/', req => {
+    const { provider } = req.params;
+    throw new NotFoundError(`Unknown auth provider '${provider}'`);
+  });
+
   router.use(
     createOidcRouter({
       tokenIssuer,