How to pass group vars to role in playbook?

[Dan]

When test_var is defined inside the playbook it gets passed to the role correctly. However when it's defined in group_vars/all.yml Ansilbe won't find it (undefined) and exits with an error:

fatal: [vagrant] => failed to combine variables, expected dicts but got a 'dict' and a 'unicode'

The unicode parameter is actually (Undefined,).

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# site.yml

vars:

  var_in_playbook: bar

roles:

  # this works

  - { role: test, my_role_var: "{{ var_in_playbook }}" }

  # this doesn't 

  - { role: test, my_role_var: "{{ var_in_group_vars }}" }

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Is there any workaround to this issue?