title: TypeError: float() argument must be a string or a number
body: <p>Wrote a code earlier and forgot to save it and now I am frankenstein-ing it all together- but I can't remember what I did  for this chunk and now I'm getting an error.</p>
  
  <pre><code>writer = open(file_name, 'w')
  for i in range (len(mammal_name)):
      mammal_lbs=round((float(mammal_kg[i]))*2.2046,3)    
      brain_lbs=round((float(brain_g[i]))*.0022046,3)
      writer.write(mammal_name[i]+','+str(mammal_lbs)+','+str(brain_lbs)+'\n')
  writer.close()
  </code></pre>
  
  <p>The code works fine if I don't change what's in the list or if I remove stuff.
  But, in other parts of the code I add into the lists and it's giving me this error:</p>
  
  <blockquote>
    <p>TypeError: float() argument must be a string or a number.</p>
  </blockquote>
  
  <p>I feel like I am doing something stupid.</p>