Documented defaults seem not to work

[magic-happenz]

Hello,
I configured the defaults according to the docs in my _config.yml as:

defaults:
  - scope:
      path: ""
    values:
      image: /assets/images/image.png

However, it does not seem to set the tags accordingly. The og:image tag is just missing. If I set up

image:
    path: /assets/images/image.png

on the respective page however it works.