diff --git a/lib/types/validators/user-validators.js b/lib/types/validators/user-validators.js
--- a/lib/types/validators/user-validators.js
+++ b/lib/types/validators/user-validators.js
@@ -4,7 +4,7 @@
 
 import { policyTypeValidator } from '../../facts/policies.js';
 import { mixedRawThreadInfoValidator } from '../../permissions/minimally-encoded-raw-thread-info-validators.js';
-import { tShape, tID } from '../../utils/validation-utils.js';
+import { tShape, tID, tUserID } from '../../utils/validation-utils.js';
 import type {
   LogOutResponse,
   RegisterResponse,
@@ -34,7 +34,7 @@
 
 export const registerResponseValidator: TInterface<RegisterResponse> =
   tShape<RegisterResponse>({
-    id: t.String,
+    id: tUserID,
     rawMessageInfos: t.list(rawMessageInfoValidator),
     currentUserInfo: loggedInUserInfoValidator,
     cookieChange: tShape({