Using Circle CI Parameterized Build API for a branch with paths

Build Environment
4

I have checked, double checked and triple checked, I certainly have the branch name correct.

I have tried this:

curl -X POST https://circleci.com/api/v1.1/project/bitbucket/:username/:project/tree/feature/bogus-ci-pr-test?circle-token=:token

And this:

curl -X POST https://circleci.com/api/v1.1/project/bitbucket/:username/:project/tree/feature%2Fbogus-ci-pr-test?circle-token=:token

In both cases I get:

{
  "message" : "Branch not found"
}

Also see: Cannot trigger build on a branch with slash in name using API