LockService: NPE on delete lock

"one" unlocked by D::1
2 [ERROR] UNICAST2: couldn't deliver message [dst: B, src: A (3 headers), size=84 bytes, flags=DONT_BUNDLE]
java.lang.NullPointerException
at org.jgroups.protocols.Locking.handleDeleteLockRequest(Locking.java:505)
at org.jgroups.protocols.Locking.up(Locking.java:243)
at org.jgroups.protocols.FRAG2.up(FRAG2.java:181)
at org.jgroups.protocols.FlowControl.up(FlowControl.java:418)
at org.jgroups.protocols.FlowControl.up(FlowControl.java:400)
at org.jgroups.protocols.pbcast.GMS.up(GMS.java:870)
at org.jgroups.protocols.pbcast.STABLE.up(STABLE.java:245)
at org.jgroups.protocols.UNICAST2.handleDataReceived(UNICAST2.java:746)
at org.jgroups.protocols.UNICAST2.up(UNICAST2.java:338)
at org.jgroups.protocols.pbcast.NAKACK.up(NAKACK.java:583)
at org.jgroups.protocols.BARRIER.up(BARRIER.java:102)
at org.jgroups.protocols.VERIFY_SUSPECT.up(VERIFY_SUSPECT.java:134)
at org.jgroups.protocols.FD_SOCK.up(FD_SOCK.java:273)
at org.jgroups.protocols.MERGE2.up(MERGE2.java:208)
at org.jgroups.protocols.Discovery.up(Discovery.java:334)
at org.jgroups.protocols.TP.passMessageUp(TP.java:1091)
at org.jgroups.protocols.TP$IncomingPacket.handleMyMessage(TP.java:1646)
at org.jgroups.protocols.TP$IncomingPacket.run(TP.java:1628)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor