Example ansible error with nested when in role

gistfile1.txt

# works fine: 
- name: generate kittens to take over the world
  hosts: places_where_dogs_reside
  gather_facts: yes
  roles: 
    - { role: train-kitten-for-battle, topics: ["whiskers"], partitions: 6, replication: 2}

# does not work: 
- name: generate kittens to take over the world
  hosts: places_where_dogs_reside
  gather_facts: yes
  roles: 
    - { role: train-kitten-for-battle, when: ansible_os_family == "Debian", namespaces: ['fluffy', 'brown','cat'], topics: ["whiskers"], partitions: 6, replication: 2}