TYPO3 Core

    public function sql_free_result($res) {
        if ($this->debug_check_recordset($res) && is_object($res)) {
            return $res->free();
        } else {
            return FALSE;
        }
    }

In case of success the current implementation returns the result of $res->free(). Unfortunately free() doesn't return anything.

Solution:
Return TRUE in case of success