diff --git a/packages/builder/src/stores/portal/users.js b/packages/builder/src/stores/portal/users.js
index 206e14568b..ff9045ae7a 100644
--- a/packages/builder/src/stores/portal/users.js
+++ b/packages/builder/src/stores/portal/users.js
@@ -40,7 +40,7 @@ export function createUsersStore() {
       inviteCode,
       password,
       firstName,
-      lastName,
+      lastName: !lastName?.trim() ? undefined : lastName,
     })
   }