Exception with "not in" condition with (int-values)

// Bob wears plaid
Golfer( name == "Bob",
$bobsPosition : position not in ($fredsPosition, $joesPosition, $fredsNeighbourPosition),
$bobsColor : color == "plaid",
color != $fredsColor, color != $joesColor,
color != $fredsNeighbourColor )

yields the following exception
Exception in thread "main" java.lang.NullPointerException
at org.drools.rule.AbstractCompositeRestriction$CompositeContextEntry.updateFromTuple(AbstractCompositeRestriction.java:112)
at org.drools.common.QuadroupleBetaConstraints.updateFromTuple(QuadroupleBetaConstraints.java:178)
at org.drools.reteoo.JoinNode.assertTuple(JoinNode.java:115)
at org.drools.reteoo.SingleTupleSinkAdapter.propagateAssertTuple(SingleTupleSinkAdapter.java:20)
at org.drools.reteoo.JoinNode.assertTuple(JoinNode.java:120)
at org.drools.reteoo.SingleTupleSinkAdapter.propagateAssertTuple(SingleTupleSinkAdapter.java:20)
at org.drools.reteoo.JoinNode.assertObject(JoinNode.java:162)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:20)
at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:130)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:20)
at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:130)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:317)
at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:168)
at org.drools.reteoo.Rete.assertObject(Rete.java:168)
at org.drools.reteoo.ReteooRuleBase.assertObject(ReteooRuleBase.java:190)
at org.drools.reteoo.ReteooWorkingMemory.doInsert(ReteooWorkingMemory.java:70)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:848)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:822)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:623)
at arc42.drools.samples.golfer.GolfingExample.main(GolfingExample.java:34)