Below is a fully self-contained DRL that compiles correctly but runs into a runtime error when the LHS of rule "match" is evaluated. The "culprit" is the test "this != $c", but one would expect simply a true result from the comparison. declare Person name: String end declare Customer extends Person rating: int end declare Employee extends Person wage: int end rule initphone salience 100 when then insert( new Customer( "Joe", 100 ) ); insert( new Employee( "Paul", 2100 ) ); end rule match when $c: Customer() $e: Employee( this != $c ) then System.out.println( "c/e " + $c + " " + $e ); end Exception in thread "main" Exception executing consequence for rule "initphone" in express: [Error: incompatible types in statement: class express.Customer (compared from: class express.Employee)] [Near : {... this != $c ....}] ^ [Line: 1, Column: 1] at org.drools.runtime.rule.impl.DefaultConsequenceExceptionHandler.handleException(DefaultConsequenceExceptionHandler.java:39) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1297) at org.drools.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:1221) at org.drools.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1456) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:710) at org.drools.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:674) at org.drools.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:230) at express.Main.execute(Main.java:92) at express.Main.main(Main.java:126) Caused by: [Error: incompatible types in statement: class express.Customer (compared from: class express.Employee)] [Near : {... this != $c ....}] ^ [Line: 1, Column: 1] at org.mvel2.ast.BinaryOperation.<init>(BinaryOperation.java:84) at org.mvel2.util.CompilerTools.finalizePayload(CompilerTools.java:118) at org.mvel2.compiler.ExpressionCompiler._compile(ExpressionCompiler.java:287) at org.mvel2.compiler.ExpressionCompiler.compile(ExpressionCompiler.java:62) at org.mvel2.MVEL.compileExpression(MVEL.java:810) at org.drools.base.mvel.MVELCompilationUnit.compile(MVELCompilationUnit.java:435) at org.drools.base.mvel.MVELCompilationUnit.getCompiledExpression(MVELCompilationUnit.java:238) at org.drools.rule.constraint.MvelConstraint.createMvelConditionEvaluator(MvelConstraint.java:206) at org.drools.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:190) at org.drools.rule.constraint.MvelConstraint.isAllowedCachedRight(MvelConstraint.java:184) at org.drools.common.SingleBetaConstraints.isAllowedCachedRight(SingleBetaConstraints.java:134) at org.drools.reteoo.JoinNode.propagateFromRight(JoinNode.java:156) at org.drools.reteoo.JoinNode.assertObject(JoinNode.java:148) at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:59) at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:235) at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:240) at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:350) at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:311) at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:903) at org.drools.base.DefaultKnowledgeHelper.insert(DefaultKnowledgeHelper.java:192) at org.drools.base.DefaultKnowledgeHelper.insert(DefaultKnowledgeHelper.java:186) at express.Rule_initphone_aea184dfceb14326841c37daadabfdbc.defaultConsequence(Rule_initphone_aea184dfceb14326841c37daadabfdbc.java:8) at express.Rule_initphone_aea184dfceb14326841c37daadabfdbcDefaultConsequenceInvokerGenerated.evaluate(Unknown Source) at express.Rule_initphone_aea184dfceb14326841c37daadabfdbcDefaultConsequenceInvoker.evaluate(Unknown Source) at org.drools.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1287) ... 7 more