The API response contains a tax amount even when it's a response saying the request failed, so this is intended to see whether it's returning a number (which it does as a string). I think the object behind amount should know what to do when it's passed an empty string and return 0 for the amount. Otherwise I'll be writing logic like the following to handle that case.

```ruby
if success?
  raise InvalidResponseError, "invalid amount"
else
  @total_tax = nil
end
```

which isn't very nice at all.