Problem With MultiSelector - Cant save to database

Hello,

I have create a multi selector form with the following code

$select =& $mform->addElement('select', 'stuff', 'Assign Stuff', array(1,2,3,4,5));
$select->setMultiple(true);

The stuff column exists in my Table and it is in text type. When i press the save button it doesnt save anything to that column and i really dont know the reason.

I tried to do the same with non-multiselector data and it worked perfectly (it saves to database). 

Also my all other data  (text,textarea fields) are saved with success into database.

What i am doing wrong?

Thank you