Usermanager generate email confirmation token sometimes returns null

Why is it that sometimes the _userManager.GenerateEmailConfirmationTokenAsync(user); returns null. It's very rare to happen but it is happening and I need to know the reason behind it.

This is my current configuration for Identity

builder.Services.AddIdentity<ApplicationUser, ApplicationRole>(options =>
   {
       options.SignIn.RequireConfirmedAccount = true;
       options.Password.RequireDigit = false;
       options.Password.RequireNonAlphanumeric = false;
       options.Password.RequireUppercase = false;
       options.Password.RequiredLength = 8;
       options.Password.RequiredUniqueChars = 0;
   })
    .AddErrorDescriber<LocalizedIdentityErrorDescriber>() // Add the localization for identity validation messages
    .AddEntityFrameworkStores<ApplicationDbContext>()
    .AddDefaultTokenProviders();