[please help]this SQL syntax make me crazy

[eluser]rikes_denis[/eluser]
i'm trying to insert a value to my database, everything is fine except the inserting thing. this is my controller function

Code:

$items = $this->okapi_termmodel->get_term()->result();
        var_dump($items);
        $insertIDF = 'INSERT INTO okapi_term(idf) VALUES(?)';
        foreach($items as $item) {
            $this->db->query($insertIDF, $this->idf($item->term));
            }

when i var_dump the $items, it shows all the database perfectly.
but in the end it return this error

A Database Error Occurred


Error Number: 1062

Duplicate entry '' for key 'term'

INSERT INTO okapi_term(idf) VALUES(-2.1972245773362)

can someone help me???
thanx b4