NPE in org.drools.core.common.TupleSetsImpl.setNextTuple

In our integration tests which were perfectly running with drools 6.3.0.Final, now we have failures with the following exception during the rules evaluation:

java.lang.NullPointerException: null
at org.drools.core.common.TupleSetsImpl.setNextTuple(TupleSetsImpl.java:349) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.common.TupleSetsImpl.removeUpdate(TupleSetsImpl.java:205) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.common.TupleSetsImpl.addDelete(TupleSetsImpl.java:110) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.reteoo.QueryElementNode$UnificationNodeViewChangedEventListener.rowRemoved(QueryElementNode.java:444) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.PhreakQueryTerminalNode.doLeftDeletes(PhreakQueryTerminalNode.java:154) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.PhreakQueryTerminalNode.doNode(PhreakQueryTerminalNode.java:46) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.RuleNetworkEvaluator.innerEval(RuleNetworkEvaluator.java:282) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.RuleNetworkEvaluator.evalStackEntry(RuleNetworkEvaluator.java:198) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.RuleNetworkEvaluator.outerEval(RuleNetworkEvaluator.java:141) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.RuleNetworkEvaluator.evaluateNetwork(RuleNetworkEvaluator.java:94) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.RuleExecutor.reEvaluateNetwork(RuleExecutor.java:194) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.phreak.RuleExecutor.evaluateNetworkAndFire(RuleExecutor.java:73) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:970) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.common.DefaultAgenda.fireLoop(DefaultAgenda.java:1312) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1251) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.impl.StatefulKnowledgeSessionImpl.internalFireAllRules(StatefulKnowledgeSessionImpl.java:1364) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1355) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:1346) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.command.runtime.rule.FireAllRulesCommand.execute(FireAllRulesCommand.java:109) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.command.runtime.rule.FireAllRulesCommand.execute(FireAllRulesCommand.java:36) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.command.runtime.BatchExecutionCommandImpl.execute(BatchExecutionCommandImpl.java:137) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.command.runtime.BatchExecutionCommandImpl.execute(BatchExecutionCommandImpl.java:51) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]
at org.drools.core.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:254) ~[drools-core-6.5.0.Final-redhat-2.jar:6.5.0.Final-redhat-2]