getRateLimit() at https://github.com/kohsuke/github-api/blob/master/src/main/java/org/kohsuke/github/GitHub.java#L228 fails for GitHub Enterprise with a 404 status code, body of

{
  "message": "No rate limit for white listed users"
}

GitHub Enterprise users are considered white listed and so don't have rate limits.

See also janinko/ghprb#127