title: How can I avoid "ValueError: invalid literal for int() with base 10: ''"?
body: <p>I have been working on a program that is meant to encrypt messages inputted by the user. After the encryption process is finished, I would like the program to prompt the user to choose whether they want to encrypt another message or not.</p>
  
  <pre><code>    option2 = int(input('Would you like to encrypt another message? (Yes = 1 and No = 2)'))
      while option2 not in [1, 2]:
  
          print 'Please type 1 or 2.'
          option2 = int(raw_input())
      while True:
          option2 = int(raw_input())
          if option2 == 1:
              option1 = int(input('Which encryption method would you like to use? 1 = Across (NOPQ ...) and 2 = Backwards (ZYXW ...)'))
      while True:
          option2 = int(raw_input())
          if option2 == 2:
              break
  </code></pre>
  
  <p>This code results in </p>
  
  <blockquote>
    <p>"ValueError: invalid literal for int() with base 10: ''"</p>
  </blockquote>
  
  <p>an error I have never before encountered. How do I fix this?</p>