Randomly NoneType object is not callable error
<p>Recently I've got this problem in my application:</p>

<blockquote>
  <p>File "main.py", line 31, 
  in File "app.pyc", line 205, in run
  TypeError: 'NoneType' object is not callable"</p>
</blockquote>

<p>My code:</p>

<pre><code>xml = EXML()
for pid, product in store.products.items():
    xml.addRow()
    xml.addCell((product['name']).encode('utf-8'), "String")
    xml.addCell((product['symbol']).encode('utf-8'), "String")
    xml.addCell((product['brand_name']).encode('utf-8'), "String") # line 205
    xml.addCell(str(product['price']), "String")
</code></pre>

<p>Python 2.7 32-bit</p>

<p>It's wired, because this showed up after ~1000 iterations, with out any previus problem.
This application scans online store to get current prices.
Firstly I thought that somewhere I missed someting, and as result there is <code>None.encode('utf-8')</code>, but no, and "".encode('utf-8') seems to work. Moreover, I can't reproduce this error on testing site, just sometimes shows up while hard-working with ~2500 products.
What are possible other sources of this error?</p>
