Asking for COV Subscriptions ended in NPE:

[BACnet4J transport] ERROR com.serotonin.bacnet4j.transport.DefaultTransport - Error during receive: NPDU [from=Address [networkNumber=0, macAddress=[c0,a8,0,83,ba,c0]], linkService=null, queue=[]]
java.lang.NullPointerException
	at com.serotonin.bacnet4j.type.Encodable.write(Encodable.java:234)
	at com.serotonin.bacnet4j.type.constructed.ObjectPropertyReference.write(ObjectPropertyReference.java:58)
	at com.serotonin.bacnet4j.type.constructed.BaseType.write(BaseType.java:41)
	at com.serotonin.bacnet4j.type.Encodable.write(Encodable.java:234)
	at com.serotonin.bacnet4j.type.constructed.CovSubscription.write(CovSubscription.java:57)
	at com.serotonin.bacnet4j.type.constructed.SequenceOf.write(SequenceOf.java:62)
	at com.serotonin.bacnet4j.type.constructed.BaseType.write(BaseType.java:41)
	at com.serotonin.bacnet4j.type.Encodable.writeEncodable(Encodable.java:323)
	at com.serotonin.bacnet4j.service.acknowledgement.ReadPropertyAck.write(ReadPropertyAck.java:88)
	at com.serotonin.bacnet4j.transport.DefaultTransport.sendConfirmedResponse(DefaultTransport.java:752)
	at com.serotonin.bacnet4j.transport.DefaultTransport.incomingConfirmedRequest(DefaultTransport.java:704)
	at com.serotonin.bacnet4j.transport.DefaultTransport.receiveAPDU(DefaultTransport.java:531)
	at com.serotonin.bacnet4j.transport.DefaultTransport.receiveImpl(DefaultTransport.java:463)
	at com.serotonin.bacnet4j.transport.DefaultTransport.run(DefaultTransport.java:404)
	at java.lang.Thread.run(Thread.java:745)
```