'PingCompletedEventArgs.reply' is null

Question

What causes the Reply to be null? I am building a game via Unity3D.

The code works on my PC - ping successfully completes, and I can access the reply.RoundtripTime. But when building and running on iOS (iPhone), the reply remains null, even though the _respose.Cancelled is false and Error is null? The phone is connected to internet.

class MyPing{
    public SNNI.Ping ping{ get; private set;} = null;
    public Ip_Array ip{ get; private set;} = null;
    public SNNI.PingCompletedEventArgs _response{ get; private set;} = null;

    public bool _isDone{ 
        get{ return _response != null; }
    }

    public MyPing( Ip_Array ip, int ms_timeout=1000 ){
        this.ip = ip;
        ping = new SNNI.Ping();
        ping.PingCompleted += (object sender, SNNI.PingCompletedEventArgs r)=>{  _response = r; };

        System.Net.IPAddress addr = new System.Net.IPAddress( ip.ip );
        ping.SendAsync(addr, ms_timeout, null);
    }
}

.

//somewhere else:
p = new MyPing( new Ip_Array("0.0.0.0") );//<--actual ip instead of this placeholder

.

//later, checking every frame of the game:
if(p._isDone == false){  continue;  } 
if(p._response.Cancelled || p._response.Error != null){ continue; }
else{
    Debug.Log(p._response.reply.Status);//throws exception because .reply is somewhow 'null'
} 

Interestingly there is an example on using PingCompletedEventArgs, specifically the DisplayReply() method. In there they check if reply==null but never explain why it might be null...