Dbforge alter add column always create NOT NULL field

I don't know if it is a bug but I add these lines when I call for add_column or _alter_table() now:

PHP Code:

if (substr($sql, -8, 8) == "NOT NULL") {
    $sql = substr_replace($sql, '', -8, 8);
} 


Remove the last NOT NULL text in CodeIgniter-built query