Incorrect error message when value is NaN

```
convert(value, {
      from: 'USD',
      to: 'JPY',
      base: 'USD',
      rates: exchangeRates,
    })
```

when value is NaN it throw ``rates` object does not contain either `from` or `to` currency!`  . Which seems unrelated.

Should not it throw error related to value is NaN instead?