Entity Framework Core API return table with .Include

I have a Model with ICollection:

public class Blog
{
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    public ICollection<Posts> Posts { get; set; }
}

This works with MVC:

public async Task<IActionResult> Index()
{
    return View(await _context.Blog.Include(l => l.Posts).ToListAsync());
}

I try to use API-Controller:

[Route("api/[controller]")]
[ApiController]
....
[HttpGet]
public IEnumerable<Blog> GetAll()
{
    return _context.Blog.Include(l => l.Posts).ToList();
}

This generate a error:

SyntaxError: JSON.parse: end of data after property value in object at line 1 column 44 of the JSON data

How to return a multidimensional Json with Blogs and for every Blog entry all Posts?