something causing a server error (500)

I have been battling this thing for a few weeks. I kept thinking the problem with the server error was my my js code running in the browser. But it seems that the CI backend is the problem. 

PHP Code:

       $table = array();
        if ($this->input->is_ajax_request()) {
            $json = $this->input->raw_input_stream;
            $json = json_decode($json);
            $email_key = $json->email;
       //     $sql="Select campaign FROM employees WHERE employee_email=?";
        //    $this->db->query($sql,array(1,$email_key));
        //    $camp = $this->db->get()->result();
            $query=$this->db->get_where('employees', array('employee_email'=>$email_key))->result();
            if ($query->num_rows() > 0)   <<<<<This is the line that causes the server error
            {
                $campaign=$query->result();
            }
            else
            {
                $this->db->close();  //no campaign for this guy
                $this->output
                    ->set_content_type('application/json')
                    ->set_output(json_encode("user not found"));
                return;
            } 


Something is wrong with the num_rows call. The get_where seems to work OK. What am I missing here?

proof that an old dog can learn new tricks