Content length is -1 on file downloads

[bertold]

We are using the call sequence such as :

request := client.R().SetOutputFile(targetFilename)
...
resp, err := request.Get(url)

and noticed the resp.ContentLength field is always -1. I could not see in the code that the library is setting this response header in case the isSaveResponse flag on the request is set. Am I reading this wrong? Thank you.