java.lang.NoClassDefFoundError for DRL function in BRMS 6

Issue

• I get NoClassDefFoundError which says Drools cannot find a class for my DRL function

java.lang.NoClassDefFoundError: mypackage/Myfunction
    at org.drools.core.common.ProjectClassLoader.tryDefineType(ProjectClassLoader.java:158)
    at org.drools.core.common.ProjectClassLoader.loadType(ProjectClassLoader.java:149)
    at org.drools.core.common.ProjectClassLoader.loadClass(ProjectClassLoader.java:119)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    at ConditionEvaluator1cdc22a152234d0abcd3a7255474435f.evaluate(Unknown Source)
    at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:217)
    at org.drools.core.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:174)
    at org.drools.core.reteoo.AlphaNode.assertObject(AlphaNode.java:134)
    at org.drools.core.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:502)
    at org.drools.core.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:387)
    at org.drools.core.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:288)
    at org.drools.core.reteoo.EntryPointNode.assertObject(EntryPointNode.java:260)
    at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:360)
    at org.drools.core.common.NamedEntryPoint.insert(NamedEntryPoint.java:279)
    at org.drools.core.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:1148)
    at org.drools.core.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:1092)
    at org.drools.core.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:308)