Why rescue nil is bad

rescue_nil_bad.rb

class Category < ActiveRecord::Base

  def ask_question(question)
    ## Code to check some other system for the answer, Redis or Memcached maybe.
    raise "System is unavailable, you should turn it on"
  end

end
  
params = {category_id: 1, question: "Why did the quick brown fox jump over the lazy dog?"}

category = Category.find(params[:category_id])
answer = category.ask_question(params[:question]) rescue nil

if answer
  ## Do something with the answer
else
  ## Do something else
end