This is an annoying glitch in the extension mechanism: because a CustomServiceTask is effectively nothing else than a ServiceTask with a set of CustomProperty fields applied, the BPMN validator will attempt to validate it as a regular ServiceTask. This will fail because, for instance, the class property of the underlying ServiceTask hasn't been set. For that reason, at present, a user has to uncheck the BPMN validator if any CustomServiceTasks are used. This should be changed so the default is to first check whether the node is really a CustomServiceTask, and if so, to skip regular validations for it. That way the BPMN validator can be left active when CustomServiceTasks are used.