Update issue (Query Builder Class)

For some reason I'm getting extra back ticks in my update statement.

       $this->db->where('table2acct_id', $acct_id);
       $this->db->where('table1id', $id);
       $this->db->where('enabled', 1);
       $this->db->where('checkin_close_time >= UTC_TIMESTAMP()');
       $this->db->update("table1 JOIN table2 ON table1id= table2id JOIN table3 ON table2id=table3id", $update_data);

The issue is in the update statement. The query returns:
UPDATE `table1 JOIN table2 ON table1id= table2id JOIN table3 ON` `table2id=table3id` SET ...

For some reason there is an extra back tick between my last ON statement.

Thanks in advance.