where_in method unexpected query

Hi,
I have a query like this

Code:

$this->db->where_in('chat_session', $latest_sessions)->update('chat_state', $update_state);

which generating following result

UPDATE `chat_state` SET `status` = 'exit'
WHERE `chat_session` IN('55_6_11,30_6_2')

But I need following query
UPDATE `chat_state` SET `status` = 'exit'
WHERE `chat_session` IN('55_6_11','30_6_2')

I have tried following still not working

Code:

$this->db->where_in('chat_session', $latest_sessions,NULL)->update('chat_state', $update_state);

and

Code:

$this->db->where_in('chat_session', $latest_sessions,FALSE)->update('chat_state', $update_state);

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