php create order can not add lines userfields

when i delete

$vOrder->LInes->UsersFields->Fields->item("U_xxx")->Value = '';

$vOrder->LInes->UsersFields->Fields->item("U_xxx")->Value = '';

$vOrder->LInes->UsersFields->Fields->item("U_xxx")->Value = '';

it worked; but with these the program have no respone , please help