gitlab-ci-linter prints "KO" with no details

I suspect that this may be happening because the API invocation fails, but I don't see any way to tell, and the -verbose option doesn't show me what's happening with the server. Here's what I see:

$ gitlab-ci-linter -verbose -verbose gitlab-ci.yml
gitlab-ci.yml used as gitlab-ci.yml file.
Settings:
  directoryRoot: /Users/rpg/projects/scepter/shop-mcts
  gitlabCiFilePath: /Users/rpg/projects/scepter/shop-mcts/gitlab-ci.yml
Checking 'https://git.sift.net' (using 'https://git.sift.net/api/v4/ci/lint')...
Url 'https://git.sift.net' validated
API url found: https://git.sift.net
Validating gitlab-ci.yml...
Querying https://git.sift.net/api/v4/ci/lint...
gitlab-ci.yml KO

Part of the reason I believe that this is an API issue, rather than something wrong with my YAML file is that this YAML file is marked as valid if I paste it into the pipelines editor on the GitLab install.

Is there any way to dump the HTTP request and response to see what might be wrong?