.attach() Has an error in firefox browser

I got an error in Firefox browser.
Here is the error:
"Error: Argument 2 of FormData.append does not implement interface Blob."

``` javascript
request
.post(url)
.attach('file', image, 'file')
.end(function(err, res) { ... })

```

here the image object can be null. The file input is optional. It raise error in Firefox browser when the file object is null. However, in Chrome browser it works fine.