Django migrate的时候报错

migration可以，但执行migrate的时候就报错

models.py

 class Category(models.Model):
    name = models.CharField('名称', max_length=16)


class Tag(models.Model):
    name = models.CharField('名称', max_length=16)


class Article(models.Model):
    title = models.CharField(max_length=32, default='Title')
    content = models.TextField(null=True)
    pub_time = models.DateTimeField(auto_now_add=True)
    author = models.CharField(max_length=16, default='Author')
    category = models.ForeignKey(Category, verbose_name='分类', default='Category')
    tags = models.ManyToManyField(Tag, verbose_name='标签')

    def __unicode__(self):
        return self.title


class Comment(models.Model):
    blog = models.ForeignKey(Article, verbose_name='博客')

    name = models.CharField('称呼', max_length=16)
    email = models.EmailField('邮箱')
    content = models.CharField('内容', max_length=140)

    created = models.DateTimeField('发布时间', auto_now_add=True)

下面是报错信息

感觉可能是版本问题但是又不知道是哪的错，把Category的语句注释了也不行，求大神帮忙解答，感谢！