appending record instead of replacing field

I am trying to update one field of a table.

PHP Code:

       $userid=$this->session->userdata('userid');
        $campaign=$this->session->userdata('campaign');
        $sql="SELECT * FROM users WHERE email = ?";
        $query = $this->db->query($sql,$userid);
        $row = $query->row();
        $iRc=$this->db->replace('users',array('campaign'=> $campaign)); 


I am finding that this is appending a new record onto the end of the table rather than replacing the campaign field if the userid matches. This is the table structure. What is the problem?

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