In Modules::define_module(), the JVM is checking if the module handle being passed is a subclass of java.lang.reflect.Module. java.lang.reflect.Module is a final class and can not be subclassed. It instead should check if the module handle is a class of java.lang.reflect.module.

  Handle module_handle(THREAD, JNIHandles::resolve(module));
  if (!java_lang_reflect_Module::is_subclass(module_handle->klass())) {
    THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(),
              "module is not a subclass of java.lang.reflect.Module");
  }