比较Guid时出现异常

我正在尝试获取包含具有特定ID的站点的所有类别的列表。下面是我稍微修改过的脚手架生成方法。

        [HttpGet("Categories/{id}")]
        public async Task<IActionResult> GetCategoriesSite([FromRoute] Guid id)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            // This line throws the error
            var categories = await _context.Categories.Where(o => o.Site.Id.Equals(id)).ToListAsync();

            if (categories == null)
            {
                return NotFound();
            }

            return Ok(categories);
        }

不幸的是,当我运行此程序时,它会引发以下错误:

System.ArgumentException: Expression of type 'System.Nullable`1[System.Guid]' cannot be used for parameter of type 'System.Guid' of method 'Boolean Equals(System.Guid)'

实体非常简单:

    public class Category
    {
        public Guid Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }

        public Site Site { get; set; }
    }

    public class Site
    {
        public Guid Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }

        public ICollection<Category> Categories { get; set; }
    }

我会做错什么?