mysqli update record returns error

[eluser]Atasa[/eluser]
Anyone had this before?
What I do is

Code:

$where = $idColumn . " = " . $idValue;
$str = $this->db->update_string($tableName, $data, $where);
if ($this->db->query($str) === TRUE)
return $idValue;

Returns:

An Error Was Encountered
Error Number: 0
UPDATE news_categories SET PARENT_CATEGORY_ID = '-1', CATEGORY_TITLE = 'TEST2' WHERE CATEGORY_ID = 1

to my surprising eyes The Update happens but I couldn't figure out from where this error is coming from.

I use mysqli driver and this error it is driving me nuts!