openpyxl generates broken workbook
<p>I am using openpyxl (2.3.5). 
When I create a new workbook and open it with Microsoft Excel, it pops up an error saying: </p>

<blockquote>
  <p>"We found a problem with some content in 'pourquoi.xlsx'. Do you want us to try to recover as much as we can? If you trust the source of this workbook, Click Yes"</p>
</blockquote>

<p>When I click 'Yes', my file is repaired.</p>

<p>Here is the simple code :</p>

<pre><code>&gt;&gt;&gt; import openpyxl                  
&gt;&gt;&gt; wb = openpyxl.Workbook()                     
&gt;&gt;&gt; wb.save('pourquoi.xlsx')                     
</code></pre>