diff --git a/app/controllers/api/users.php b/app/controllers/api/users.php
index 35c9f2497..556efebe3 100644
--- a/app/controllers/api/users.php
+++ b/app/controllers/api/users.php
@@ -31,7 +31,7 @@ App::post('/v1/users')
     ->label('sdk.response.code', Response::STATUS_CODE_CREATED)
     ->label('sdk.response.type', Response::CONTENT_TYPE_JSON)
     ->label('sdk.response.model', Response::MODEL_USER)
-    ->param('userId', '', new CustomId(), 'Unique Id. Passing the string `unique()` will auto generate your ID.')
+    ->param('userId', '', new CustomId(), 'Unique Id. Choose your own unique ID or pass the string `unique()` to auto generate it. Valid chars are a-z, A-Z, 0-9, and underscore. Can't start with a leading underscore. Max length is 36 chars.')
     ->param('email', '', new Email(), 'User email.')
     ->param('password', '', new Password(), 'User password. Must be between 6 to 32 chars.')
     ->param('name', '', new Text(128), 'User name. Max length: 128 chars.', true)