userManager.FindByIdAsync returns NULL

Question

I dont know why my userManager.FindByIdAsync returns a NULL value when a trying reset my password.

public async Task<bool> ResetPasswordByAdmin(string userId, string newPassword)
        {
            try
            {
                var user = await _userManager.FindByIdAsync(userId);
                string token = await _userManager.GeneratePasswordResetTokenAsync(user);
                var identityResult = await _userManager.ResetPasswordAsync(user, token, newPassword);
                return identityResult.Succeeded;
            }
            catch (Exception)
            {
                throw;
            }
        }

EDIT 01 (FindByIdAsync):

    public virtual Task<TUser> FindByIdAsync(string userId);
    //
    // Summary:
    //     Retrieves the user associated with the specified external login provider and
    //     login provider key.
    //
    // Parameters:
    //   loginProvider:
    //     The login provider who provided the providerKey.
    //
    //   providerKey:
    //     The key provided by the loginProvider to identify a user.
    //
    // Returns:
    //     The System.Threading.Tasks.Task for the asynchronous operation, containing the
    //     user, if any which matched the specified login provider and key.