title: TypeError: "NoneType" object is not iterable
body: <p>I know that this error message 'TypeError: 'NoneType' object is not iterable' means that there is None is the data. However, I am looking through all of my list and there is no part which has a no value element. 
  This is the section of my code that correlates to my issue. </p>
  
  <pre><code>def printList(heights):
      print(" ".join(str(h) for h in heights))
  
  
  def nextYear(heights):
      next_heights = []
      for i in range(len(heights)):
          next_heights.append(heights[i] + 5)
          i += 1
      print(" ".join(str(h) for h in next_heights))
  
  #main routine
  
  heights = [33, 45, 23, 43, 48, 32, 35, 46, 48, 39, 41]
  printList(heights)
  print("Predicted heights after a year")
  printList(nextYear(heights))
  </code></pre>
  
  <p>This is my codes output:</p>
  
  <pre><code>33 45 23 43 48 32 35 46 48 39 41
  Predicted heights after a year
  38 50 28 48 53 37 40 51 53 44 46
  Traceback (most recent call last):
    File "/Users/ellarendell/Desktop/test.py", line 17, in &lt;module&gt;
      printList(nextYear(heights))
    File "/Users/ellarendell/Desktop/test.py", line 2, in printList
      print(" ".join(str(h) for h in heights))
  TypeError: 'NoneType' object is not iterable
  </code></pre>
  
  <p>I want my code to do the same output without the error message.
  Do you know what part of the list could have 'None'?
  Thank you :)</p>