How to use with_options for conditional validation

How can I use with_options for conditional validation ?

My code is

with_options if: (AppUser::User.creator=="is_admin") do |admin|
  admin.validates :first_name, :presence => true
  admin.validates :last_name, :presence => true
end

I have already set creator method in application controller.

before_action :set_global_user

def set_global_user
  if current_admin
    AppUser::User.creator= "is_admin"
  elsif current_user
    AppUser::User.creator= "is_user"
  else
    AppUser::User.creator=nil
  end
end

but I am getting

undefined method `validate' for false:FalseClass

what is wrong with this code.