IArchive.isSourceUploadAllowed() requires existing sourcepackagename

I want to see if someone is allowed to upload to a given PPA, because of bug 814725 I can't just upload and see what the error is, I have to look before I leap.

IArchive.isSourceUploadAllowed() looks like it might be relevant, since I'm interested in seeing whether that person is allowed to upload source. However, when I ask if I am allowed to upload the source package 'whatever' to my own PPA for the current release of Ubuntu, I get an error:

>>> ppa.isSourceUploadAllowed(person=me, distroseries=ubuntu.current_series, sourcepackagename='whatever')
Traceback (most recent call last):
...
NotFound: HTTP Error 404: Not Found
Response headers:
---
...
status: 404
...
---
Response body:
---
No such source package: 'whatever'.
---