为什么选择OWLReasoner.getSubclasses(...)返回http://www.w3.org/2002/07/owl#Nothing？

Question

下面是我的本体子类请求的一部分，使用JAVA-7和owlapi库：

import org.semanticweb.owlapi.reasoner.OWLReasoner;
import org.semanticweb.owlapi.reasoner.OWLReasonerFactory;
import org.semanticweb.owlapi.reasoner.ConsoleProgressMonitor;
import org.semanticweb.owlapi.reasoner.OWLReasonerConfiguration;
...
...
OWLReasonerFactory reasonerFactory = new StructuralReasonerFactory();
ConsoleProgressMonitor progressMonitor = new ConsoleProgressMonitor();
OWLReasonerConfiguration config = new SimpleConfiguration(myconfiguration);
OWLReasoner reasoner = reasonerFactory.createReasoner(myontology, config);
Set<OWLClass> subclasses = reasoner.getSubClasses(myClazz, true).getFlattened();

这是我的问题：

为什么子类，什么OWLReasoner.getSubClasses(...)方法返回，包含myClazz的所有子类，但始终还添加带有URI的OWLClass http://www.w3.org/2002/07/owl#Nothing？我没有给这个类下任何定义。

提前谢谢。