title: Why does my code produce TypeError: 'NoneType' object is not iterable
body: <p>I know this error has been asked several times, but I am stuck with my code for why it keep returning TypeError</p>
  
  <pre><code>def merge( s1, s2):
      if len(s1) == 0:
          return s2[:]
      if len(s2) == 0:
          return s1[:]
      minElm = []
      if s1[0] &lt;= s2[0]:
          minElm.append( s1.pop(0) )
      else:
          minElm.append( s2.pop(0) )
      return minElm.extend( merge(s1[:], s2[:] ))
  
  list1 = [1,3,5,7,9]
  list2 = [2,4,6,8]
  
  merged = merge( list1[:], list2[:] )
  print(merged)
  </code></pre>
  
  <p>Basically, I want to merge <em>two already sorted lists (ASC)</em> into a <em>single sorted list</em> (ASC) using recursive method. I'm positive that my logic is correct, I just cannot get why am I getting <code>TypeError: 'NoneType' object is not iterable</code></p>
  
  <p>Why am I getting the <code>TypeError</code>?</p>