diff --git a/packages/builder/src/stores/portal/users.js b/packages/builder/src/stores/portal/users.js
index 1510207604..0f6636fbdc 100644
--- a/packages/builder/src/stores/portal/users.js
+++ b/packages/builder/src/stores/portal/users.js
@@ -34,7 +34,7 @@ export function createUsersStore() {
       inviteCode,
       password,
       firstName,
-      lastName,
+      lastName: !lastName?.trim() ? undefined : lastName,
     })
   }