AE Refresh behaviour|Classic OPC: DA, A&E, HDA, XML-DA, etc.|Forum|OPC Foundation

Hello, could you help me to understand the Refresh() process when there were not any active nor unacknowledged condition on AE server?
Should the AE server send an empty list of EventNotifications with Refresh and LastRefresh flags true?

I get some mixed result with a customer’s server. The server sometimes send nothing, and sometimes send empty pEvents to  OpcCom.Ae.Subscription.OnEvent(

int hClientSubscription,
int bRefresh,
int bLastRefresh,
int dwCount,
OpcRcw.Ae.ONEVENTSTRUCT[] pEvents)

which will cause exception:

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=OpcNetApi.Com
StackTrace:
at OpcCom.Ae.Subscription.Callback.OnEvent(Int32 hClientSubscription, Int32 bRefresh, Int32 bLastRefresh, Int32 dwCount, ONEVENTSTRUCT[] pEvents) in xxxxxxxxxx\NetCore\OpcNetApi.Com\Ae\OpcCom.Ae.Subscription.cs:line 523

This exception was originally thrown at this call stack:
OpcCom.Ae.Subscription.Callback.OnEvent(int, int, int, int, OpcRcw.Ae.ONEVENTSTRUCT[]) in OpcCom.Ae.Subscription.cs