title: Django refuses to accept my one-off default value for FloatField
body: <p>I have a class and I'm trying to add a new <code>FloatField</code> to it. Django wants a default value to populate existing rows (reasonable). However, it refuses to accept any value I give it.</p>
  
  <pre><code>You are trying to add a non-nullable field 'FIELDNAME' to CLASS without a default; we can't do that (the database needs something to populate existing rows).
  Please select a fix:
   1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
   2) Quit, and let me add a default in models.py
  Select an option: -1
  Please select a valid option: -1
  Please select a valid option: -1
  Please select a valid option: float(-1)
  Please select a valid option: -1.0
  Please select a valid option: float(-1.0)
  Please select a valid option: 
  </code></pre>
  
  <p>How do I get it to accept my value?</p>