sql error lost in transactions

The following code outputs 0 instead of the right db error code.

$db->sql_return_on_error(true);

$db->sql_transaction('begin');

if (!$db->sql_query('this will fail')) {

    $error = $db->sql_error();

    echo $error['code'];

}

sql_query calls sql_error in case of an error which makes silently a rollback without preserving the reason for the error. (Tested with mysqli layer only)