This seems to be a bug in Drools, a regression introduced into 5.5.0; it compiles in 5.[34].0 The rule uses the custom operator supersetOf, both operands are java.util.Collection. //Results in a null pointer exception: Error: java.lang.NullPointerException $test2: ArrayList() from collect (DummyTest (sentence.intentions supersetOf external.intentions)) The stack trace: Exception in thread "main" java.lang.NullPointerException at org.drools.rule.builder.MVELConstraintBuilder.buildVariableConstraint(MVELConstraintBuilder.java:90) at org.drools.rule.builder.PatternBuilder.addConstraintToPattern(PatternBuilder.java:823) at org.drools.rule.builder.PatternBuilder.addConstraintToPattern(PatternBuilder.java:736) at org.drools.rule.builder.PatternBuilder.buildRelationalExpression(PatternBuilder.java:721) at org.drools.rule.builder.PatternBuilder.buildExpression(PatternBuilder.java:673) at org.drools.rule.builder.PatternBuilder.buildCcdDescr(PatternBuilder.java:657) at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:629) at org.drools.rule.builder.PatternBuilder.processConstraintsAndBinds(PatternBuilder.java:477) at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:302) at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:123) at org.drools.rule.builder.CollectBuilder.build(CollectBuilder.java:43) at org.drools.rule.builder.CollectBuilder.build(CollectBuilder.java:32) at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:293) at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:123) at org.drools.rule.builder.GroupElementBuilder.build(GroupElementBuilder.java:67) at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:84) at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:2830) at org.drools.compiler.PackageBuilder.compileRules(PackageBuilder.java:970) at org.drools.compiler.PackageBuilder.compileAllRules(PackageBuilder.java:879) at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:871) at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:466) at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:694) at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:51) at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:40) at hezi.Main.setup(Main.java:77) at hezi.Main.main(Main.java:129) Cheers -W On 03/08/2013, hezis <hezis(a)delegatecom.com&gt; wrote: