"Unable to resolve service for type ... while attempting to activate "

Прохожу сейчас туториал metanit'а по управлению ролями в Identity. Сделал всё согласно уроку, однако при попытке вызвать RolesController получаю следующую ошибку:

InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Identity.RoleManager`1[Microsoft.AspNetCore.Identity.IdentityRole]' while attempting to activate 'Diplom1.Controllers.RolesController'.

Возможно проблема в конфигурации сервисов в Startup, но у меня нет никаких идей на этот счёт.

RolesController:

public class RolesController : Controller
{
    RoleManager<IdentityRole> _roleManager;
    UserManager<User> _userManager;
    
    public RolesController(RoleManager<IdentityRole> roleManager, UserManager<User> userManager)
    {
        _roleManager = roleManager;
        _userManager = userManager;
    }
    public IActionResult Index() => View(_roleManager.Roles.ToList());

    public IActionResult Create() => View();
    [HttpPost]
    public async Task<IActionResult> Create(string name)
    {
        if (!string.IsNullOrEmpty(name))
        {
            IdentityResult result = await _roleManager.CreateAsync(new IdentityRole(name));
            if (result.Succeeded)
            {
                return RedirectToAction("Index");
            }
            else
            {
                foreach (var error in result.Errors)
                {
                    ModelState.AddModelError(string.Empty, error.Description);
                }
            }
        }
        return View(name);
    }

    [HttpPost]
    public async Task<IActionResult> Delete(string id)
    {
        IdentityRole role = await _roleManager.FindByIdAsync(id);
        if (role != null)
        {
            IdentityResult result = await _roleManager.DeleteAsync(role);
        }
        return RedirectToAction("Index");
    }

    public IActionResult UserList() => View(_userManager.Users.ToList());

    public async Task<IActionResult> Edit(string userId)
    {
        User user = await _userManager.FindByIdAsync(userId);
        if (user != null)
        {
            var userRoles = await _userManager.GetRolesAsync(user);
            var allRoles = _roleManager.Roles.ToList();
            ChangeRoleViewModel model = new ChangeRoleViewModel
            {
                UserId = user.Id,
                UserEmail = user.Email,
                UserRoles = userRoles,
                AllRoles = allRoles
            };
            return View(model);
        }
        return NotFound();
    }

    [HttpPost]
    public async Task<IActionResult> Edit(string userId, List<string> roles)
    {
        // получаем пользователя
        User user = await _userManager.FindByIdAsync(userId);
        if (user != null)
        {
            var userRoles = await _userManager.GetRolesAsync(user);
            var allRoles = _roleManager.Roles.ToList();
            var addedRoles = roles.Except(userRoles);
            var removedRoles = userRoles.Except(roles);

            await _userManager.AddToRolesAsync(user, addedRoles);
            await _userManager.RemoveFromRolesAsync(user, removedRoles);

            return RedirectToAction("UserList");
        }
        return NotFound();
    }
}

Конфигурация Identity в Startup:

services.AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(
                Configuration.GetConnectionString("DefaultConnection")));
services.AddDefaultIdentity<IdentityUser>(options => options.SignIn.RequireConfirmedAccount = true)
            .AddEntityFrameworkStores<ApplicationDbContext>();