Is JSZip#folder(name) ever null?

[Volper212]

The documentation on the website says the JSZip#folder(name) function returns:

A new JSZip (for chaining), with the new folder as root

However, the type definitions file says that it returns:

New JSZip object with the given folder as root or null

Where is the null coming from? Is it even a thing that can be returned? If not, I would appreciate if the type could be corrected. I glanced the source code of the function and saw no null in there.