lrwan module, set 8-byte MAC address through "NetDeviceContainer"

Hey, Sir,I recently encountered a problem when using ns3 and would like to ask you In the lrwan module of ns3, use

Ptr<LrWpanNetDevice> netDevice = CreateObject<LrWpanNetDevice> ();
Ptr<LrWpanMac> m_mac=CreateObject<LrWpanMac> ();
m_mac ->SetExtendedAddress(Mac64Address::Allocate ()); netDevice->SetMac(m_mac); 

statement can set an 8-byte MAC address to "LrWpanNetDevice", but use the statement "NetDeviceContainer devices;devices.Add (netDevice) ; NS_LOG_UNCOND ("MacAddress: "<< devices.Get(0)->GetAddress());" can only get a 2-byte "00:00" error MAC address. Can you tell me how to set the "SetExtendedAddress" of "LrWpanNetDevice" to get an 8-byte MAC address through "NetDeviceContainer". Thank you very much!