Add listenner to button in loop error

I have 3 panel contain 20 Button/1 panel.

I try to add onclick listener to first panel but when I click on all buttons it alway print 19. I try to use local index variable but it donot work.

void Start() {

    for (int i = 0; i <20 ; i ++)
    {
             int index = i;
             arrayPanelIcon[0].transform.GetChild(index).GetComponent<Button>().onClick.AddListener( () => btnItem_click(index.ToString()));
     }

}

public void btnItem_click(string name) {

     Debug.Log("click: " + name);

}

Any help, please!