Using hub pull-request results in 404 error #3357

Command attempted:
hub pull-request

What happened:
It prompted for my username and password, then spat out the following error message:
Error creating pull request: Not Found (HTTP 404)
Not Found

More info:
This is not a new repository, nor would this be the first PR within it. I am new to this software, however, but the man pages imply that it should be using SSH at this point. The repo in question is a part of an organization with GitHub Classroom if that winds up being relevant, though I don't imagine it is.