我有一个Django项目，包含以下模型：

class Ingredient(models.Model):
    drink = models.ForeignKey(DrinkRecipe, on_delete=models.CASCADE)
    ingredient_rhum = models.ForeignKey(Bottle, on_delete=models.CASCADE, null=True, blank=True)
    ingredient_name = models.CharField(max_length=255, blank=True)
    ingredient_amount = models.DecimalField(decimal_places=1,max_digits=5, validators=[MinValueValidator(0.01)])
    

    def __str__(self):
            if self.ingredient_rhum:
                return self.ingredient_rhum
            return self.ingredient_name
        
    def get_absolute_url(self):
        return self.drink.get_absolute_url()

以及：

class Bottle(models.Model):
    bottle_name = models.CharField(max_length=255, blank=False, verbose_name='Name')
    bottle_slug = models.SlugField(unique=True, blank=True, null=True)
    bottle_info = HTMLField(blank=False, verbose_name='Information about the bottle')
    bottle_tastingnotes_text = HTMLField(verbose_name='Tasting description', blank=False)
    bottle_tasting_notes = HTMLField(verbose_name='Tasting notes', blank=False)
    bottle_tastingnotes_finish = HTMLField(verbose_name='Finish',blank=False)
    bottle_image = models.ImageField(upload_to=image_upload_handler)
    sorting_number = models.IntegerField()
    bottle_abv = models.DecimalField(max_digits=3,decimal_places=1, verbose_name='ABV', validators=[MinValueValidator(0.01)])
    img_location = 'bottles'
    
    def __str__(self):
        return self.bottle_name
    
    class Meta:
        ordering = ['sorting_number', 'bottle_name']

    def get_bottle_link(self):
        return reverse('rhumSite:drinkBottle',kwargs={'tag':self.bottle_name} )
    
    def get_edit_link(self):
        return reverse('rhumSite:edit_bottle',kwargs={'id':self.id})
    
    def save(self, *args, **kwargs):
        abv = str(self.bottle_abv)
        if not self.id:
            # Newly created object, so set slug
            self.bottle_slug = slugify(self.bottle_name + abv)

        super(Bottle, self).save(*args, **kwargs)

当我在CreateView中使用inline-modelset-factory时，一切都很好。但是：当我尝试在Admin中添加成分或尝试删除成分或其他具有Bottle模型ForeignKey的对象时，我会收到以下错误：
__str__ returned non-string (type Bottle)
我做错了什么，我该如何修复它？