less小白求助，关于extend的问题

类似这样的代码

.aaa{
  ........
  &-xxx{
    ........
  }
}
.bbb{
  &:extend(.aaa);
  ........
  &-xxx{
    &:extend(.aaa-xxx); //这里就会报错 extend '.aaa-xxx' has no matches
    ........
  }
}

请问这里应该怎么处理？