diff --git a/pbs-api-types/src/userid.rs b/pbs-api-types/src/userid.rs
index 7ee64fb0..12843c3d 100644
--- a/pbs-api-types/src/userid.rs
+++ b/pbs-api-types/src/userid.rs
@@ -101,6 +101,7 @@ pub const PROXMOX_AUTH_REALM_SCHEMA: Schema = PROXMOX_AUTH_REALM_STRING_SCHEMA.s
 #[api(
     type: String,
     format: &PROXMOX_USER_NAME_FORMAT,
+    min_length: 1,
 )]
 /// The user name part of a user id.
 ///