NPE in AbstractCompositeRestriction when using unbound variables

This occurs with a pattern such as
Cell( rowNo == $rano || == $rbno, colNo != $c1no && != $c2no, free contains $i )
where one or more of the bound variables $* are not bound in previous patterns.

Exception in thread "main" java.lang.NullPointerException
at org.drools.rule.AbstractCompositeRestriction.getRequiredDeclarations(AbstractCompositeRestriction.java:59)
at org.drools.rule.MultiRestrictionFieldConstraint.getRequiredDeclarations(MultiRestrictionFieldConstraint.java:73)
at org.drools.rule.Pattern.setConstraintType(Pattern.java:358)
at org.drools.rule.Pattern.addConstraint(Pattern.java:226)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:432)
at org.drools.rule.builder.PatternBuilder.buildConstraint(PatternBuilder.java:264)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:213)
at org.drools.rule.builder.PatternBuilder.build(PatternBuilder.java:108)
at org.drools.rule.builder.GroupElementBuilder.build(GroupElementBuilder.java:69)
at org.drools.rule.builder.RuleBuilder.build(RuleBuilder.java:78)
at org.drools.compiler.PackageBuilder.addRule(PackageBuilder.java:1203)
at org.drools.compiler.PackageBuilder.addPackage(PackageBuilder.java:629)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:278)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:451)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:28)
at org.drools.examples.sudoku.rules.DroolsUtil.readKnowledgeBase(DroolsUtil.java:33)
at org.drools.examples.sudoku.Main.<init>(Main.java:87)
at org.drools.examples.sudoku.Main.main(Main.java:68)