title: TypeError: unsupported operand type(s) for -: 'float' and 'NoneType' python
body: <p>Does anyone know why  I would be getting an error like this?! I would really appreciate it if you do, I am new to this and trying to learn but im getting really caught up in the nitty gritty of python!
  this is the error I am getting:</p>
  
  <pre><code>eError: unsupported operand type(s) for -: 'float' and 'NoneType'
  
   for test in test_set:
   person_id = test['person_id']
   place_id = test['place_id']
   rating = test['rating']
   predicted_rating = simple_nn(person_id, place_id, 5)
   #difference =  (rating- predicted_rating)
   sq_err = (rating- predicted_rating) * (rating - predicted_rating)
  
   sq_err_sum = sq_err 
  
   sq_err_sum = sq_err_sum + sq_err
  
  
  
   rmse = math.sqrt(sq_err_sum/5)
   print rmse
  </code></pre>