Triggering a pipeline on another branch for the same project

Hello all. I’m trying to trigger a pipeline on a different branch from within the same project.

The pipeline I’m trying to trigger is on the pages branch - I’d like for my GitLab Pages website to be updated on a successful build of my project on the master branch.

When my master branch pipeline finishes, it fails on the last job which is to trigger the pages branch pipeline:

update_website:
  stage: website
  rules:
    - if: '$CI_COMMIT_REF_NAME == "master"'
      when: on_success
  trigger:
    project: leavesunderfoot/nimblenote
    branch: pages

When the pipeline job is run, it fails with:

downstream pipeline trigger definition is invalid

Both the master and pages branches, each with their own .gitlab-ci.yml files are within the leavesunderfoot/nimblenote project.

From what I can see, both the project and branch values in the yml are correct.