ASP.NET Core Web API - System.AggregateException: 'Unable to cast object of type 'System.Int64' to type 'System.String''

Question

In ASP.NET Core-6 Web API, I have this code in Program.cs:

public static async Task Main(string[] args)
{
    var host = CreateHostBuilder(args).Build();

    using (var scope = host.Services.CreateScope())
    {
        var services = scope.ServiceProvider;

        try
        {
            var context = services.GetRequiredService<ApplicationDbContext>();

            if (context.Database.IsSqlServer())
            {
                await context.Database.MigrateAsync();
            }

            var userManager = services.GetRequiredService<UserManager<ApplicationUser>>();
            var roleManager = services.GetRequiredService<RoleManager<IdentityRole>>();

            ApplicationDbContextSeed.SeedData(userManager, roleManager);
        }
        catch (Exception ex)
        {
            var logger = scope.ServiceProvider.GetRequiredService<ILogger<Program>>();

            logger.LogError(ex, "An error occurred while migrating or seeding the database.");

            throw;
        }
    }

    await host.RunAsync();
} 

ApplicationUser:

public class ApplicationUser : IdentityUser
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

ApplicationDbContextSeed

public static class ApplicationDbContextSeed
{
    public static void SeedData(UserManager<ApplicationUser> userManager, RoleManager<IdentityRole> roleManager)
    {
        SeedRoles(roleManager);
        SeedUsers(userManager);
    }
    public static void SeedRoles(RoleManager<IdentityRole> roleManager)
    {
        if (!roleManager.RoleExistsAsync("Admin").Result)
        {
            IdentityRole role = new IdentityRole();
            role.Name = "Admin";
            IdentityResult roleResult = roleManager.
            CreateAsync(role).Result;
        }
    }
    public static void SeedUsers(UserManager<ApplicationUser> userManager)
    {
        if (userManager.FindByNameAsync("admin").Result == null)
        {
            ApplicationUser user = new ApplicationUser()
            {
                UserName = "admin",
                Email = "[email protected]",
                FirstName = "Firstname1",
                LastName = "Lastname1",
            };
            IdentityResult result = userManager.CreateAsync
            (user, "Admin12345").Result;

            if (result.Succeeded)
            {
                userManager.AddToRoleAsync(user, "Admin").Wait();
            }
        }

        if (userManager.FindByNameAsync("admin2").Result == null)
        {
            ApplicationUser user = new ApplicationUser()
            {
                UserName = "admin2",
                Email = "[email protected]",
                FirstName = "Firsname2",
                LastName = "Lastname2",
            };
            IdentityResult result = userManager.CreateAsync
            (user, "Admin12345").Result;

            if (result.Succeeded)
            {
                userManager.AddToRoleAsync(user, "Admin").Wait();
            }
        }
    }
}

When I run the application, I got this error:

System.AggregateException
  HResult=0x80131500
  Message=One or more errors occurred. (Unable to cast object of type 'System.Int64' to type 'System.String'.)
  Source=System.Private.CoreLib
  StackTrace:
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.Task`1.get_Result()
   at Core.Persistence.ApplicationDbContextSeed.SeedRoles(RoleManager`1 roleManager) in C:\MyApp\WebApi\Core\Persistence\ApplicationDbContextSeed.cs:line 20
   at Core.Persistence.ApplicationDbContextSeed.SeedData(UserManager`1 userManager, RoleManager`1 roleManager) in C:\MyApp\WebApi\Core\Persistence\ApplicationDbContextSeed.cs:line 15
   at WebApi.Program.<Main>d__0.MoveNext() in C:\MyApp\WebApi\Program.cs:line 46

  This exception was originally thrown at this call stack:
    [External Code]

Inner Exception 1:
InvalidCastException: Unable to cast object of type 'System.Int64' to type 'System.String'.

This is line 20:

if (!roleManager.RoleExistsAsync("Admin").Result)

This is line 15:

SeedRoles(roleManager);

The full error as displayed when I run the application is shown above. I got this while debugging. So I updated the initial one I have with this full error detail.

throw is highlighted.

How do I resolve this?

Thanks