Ansible --check option?

[helmo]

The obvious command is ansible-playbook site.yml --check --diff -vvv but fails with

[DEPRECATION WARNING]: Skipping task due to undefined Error, in the future this will be a fatal error.: 'dict object' has no attribute 'json'.
This feature will be removed in a future release. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.
fatal: [localhost]: FAILED! => {"failed": true, "msg": "with_nested requires at least one element in the nested list"}

Any hints?