createDocument() from base64 or url result in RichApi.Error: GeneralException

Question

I am trying to open an existing document from within a Word add-in. I tried:
var newDoc = context.application.createDocument(base64Image)
newDoc.open()
context.sync()
but I'm am getting an error: Uncaught (in promise) RichApi.Error: GeneralException.
When I don't pass in an argument into createDocument(), it successfully opens a blank document.

I've placed the base64Image into the current document using insertFileFromBase64 and insertInlinePictureFromBase64, so I know the base64Image file is supported.

Any other ideas?