title: Why does the type error appear if the column is clear?
body: <p>I checked <code>w_table.iloc[i,4]</code> and didn't find <code>NoneType</code> objects in it. What's the matter could it be?</p>
  
  <pre><code>check = ['word']
  for i in range(len(w_table)):
      if w_table.iloc[i, 4] != 'Null':
          if w_table.iloc[i, 4] in check:
              w_table = w_table.drop(w_table.index[i])
          else:
              check = check.append(w_table.iloc[i, 4])
          w_table.index = np.arange(len(w_table)) 
  </code></pre>
  
  <p>After executing above code I am getting following <code>TypeError</code></p>
  
  <pre><code> TypeError                                 Traceback (most recent call
   last) &lt;ipython-input-74-40b9156195fa&gt; in &lt;module&gt;()
  
         2 for i in range(len(w_table)):
         3     if w_table.iloc[i, 4] != 'Null':
         4         if w_table.iloc[i, 4] in check:
         5             w_table = w_table.drop(w_table.index[i])
         6         else:
  
   TypeError: argument of type 'NoneType' is not iterable
  </code></pre>