django admin后台插入数据保存时报 Exception Value: 'NoneType' object is not iterable？

django admin后台插入数据保存时报 Exception Value:

'NoneType' object is not iterable

class GoodsSKU(BaseModel):
'''商品SKU模型类'''
status_choices = (
(0,'上线'),
(1,'下线'),
)
type = models.ForeignKey('GoodsType',on_delete=models.CASCADE,verbose_name='商品种类')
goods = models.ForeignKey('Goods',on_delete=models.CASCADE,verbose_name='商品SPU')
name = models.CharField(max_length=20,verbose_name='商品名称')
desc = models.CharField(max_length=256,verbose_name='商品简介')
price = models.DecimalField(max_digits=10,decimal_places=2,verbose_name='商品价格')
unite = models.CharField(max_length=20,verbose_name='商品单位')
image = models.ImageField(upload_to='goods',verbose_name='商品图片')
stock = models.IntegerField(default=1,verbose_name='商品库存')
sales = models.IntegerField(default=0,verbose_name='商品销量')
status = models.SmallIntegerField(default=1,choices=status_choices,verbose_name='商品状态')

class Meta:
    db_table = 'df_goods_sku'
    verbose_name = '商品'
    verbose_name_plural = verbose_name

应该是有什么东西是空的