Non-invocable member 'NotFound' cannot be used like a method.

using Microsoft.AspNetCore.Http.HttpResults;
using Microsoft.AspNetCore.Mvc;
using RoBotAPI.Models;
using RoBotAPI.Services;

namespace RoBotAPI.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class UsersController
    {
        private readonly UsersService _usersService;
        public UsersController(UsersService usersService) =>
            _usersService = usersService;

        [HttpGet]
        public async Task<List<User>> Get() =>
            await _usersService.GetAsync();

        [HttpGet("{id}")]
        public async Task<ActionResult<User>> Get(ulong id)
        {
            var user = await _usersService.GetAsync(id);
            return user ?? NotFound();
        }
    }
}