Why UnityEngine.Ping get return time -1 in some Android Devices?

I use UnityEngine.Ping to check the network state, expect to get delay time by Ping.time.
But in some Android Devices, Ping.time always return -1.
By the way, I try to use System.Net.NetworkInformation.Ping , also failed with a InvalidOperationException.
Someone have meet the thing problem, could help.