Unable to manually trigger master-build when "except" is used.

Summary

Having following rudimentary ".gitlab-ci.yml":

stages:
    - setup
    - build
    - package
    - deploy

setup-frontend:
    stage: setup
    except:
      changes:
        - ".gitlab-ci.yml"
    script:
        - echo foo

i try to manually trigger a "master"-branch build using "Pipepines"-"Run pipeline"-Button in the WebUI.

The pipeline start fails with following error:

If i do comment out the "except:" section (lines 10-12 in https://gitlab.com/v2b1n/test/blob/master/.gitlab-ci.yml) then i can start the pipeline normally.

Steps to reproduce

Use the mentioned .gitlab-ci.yml and try to start a Pipeline on a "master"-branch using "Pipepines"-"Run pipeline"-Button.

If i do comment out the "except:" section (lines 10-12 in https://gitlab.com/v2b1n/test/blob/master/.gitlab-ci.yml) then i can start the pipeline normally.

Example Project

https://gitlab.com/v2b1n/test/pipelines

What is the current bug behavior?

The pipeline does not start.

What is the expected correct behavior?

The pipeline should start.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com.

I also was able to reproduce this behaviour in v11.11.8 of self-managed gitlab foss instance.