title: TypeError: '<' not supported between instances of 'NoneType' and 'str'
body: <p>I"m getting this error as I try to make a ttk.Combobox using the values of a set that I select from a .db file.</p>
  
  <pre><code>for row in self.sql.execute("SELECT {0} FROM Songinfo".format(self.variable1.get())):
          self.List2.append(row)
          self.seen.add(row)
  self.Option2 = ttk.Combobox(self, values=sorted(self.seen), textvariable=self.variable2)
  self.Option2.grid(row=3, column=1)
  </code></pre>
  
  <p><code>self.seen</code>, when printed out returns something like:</p>
  
  <pre><code>{('Heavy Metal',), ('Soundtrack',), ('Pop/Rock',), ('Metal',), 
   ('Alternative',), ('Alternative &amp; Punk',), ('Rock',),
   ('Pop',), ('Classical Crossover',), (None,)}
  </code></pre>
  
  <p>this is a set of genres.  I'm getting that error and I'm not sure why, it wasn't an issue till recently, any help is appreciated, thanks.</p>