I ran into the same issue: /tmp/blog.rust-lang.org$ git hub pull new Pushing nonnull-size to nonnull-size in fork Creating pull request from branch nonnull-size to rust-lang/blog.rust-lang.org:master Error: GitHub error: Validation Failed Error: HTTP Error 422: Unprocessable Entity Error: https://api.github.com/repos/rust-lang/blog.rust-lang.org/pulls Error: <httplib.HTTPMessage instance at 0x7fa068308758> Error: {"message":"Validation Failed","errors":[{"resource":"PullRequest","field":"base","code":"invalid"}],"documentation_url":"https://developer.github.com/v3/pulls/#create-a-pull-request"}
The mention of needing to specify the branch helped, because it turned out I needed to create the pull request against the gh-pages branch. It would definitely have helped if the error message explained that the "master" branch didn't exist.
