elixir - Why I get ** (EXIT) bad return value: :ok -

i need let genserver monitor task, thus, do:

genserver.call(server_pid, {:monitor_task, self()}) 

in server:

def handle_call({:monitor_task, task_pid}, _from, state)   ref = process.monitor(task_pid)   {:reply, ref, state} end 

however, error:

** (stop) exited in: genserver.call(#pid<0.768.0>, {:monitor_task, #pid<0.849.0>}, 5000)     ** (exit) bad return value: :ok 

any idea?

the handle_info callbacks calls logger need return {:noreply, state} or {:stop, reason, state}, calls logger return :ok, invalid return value callback, , caused error seeing.