Ansible: ERROR! failed to combine variables, expected dicts but got a 'dict' and a 'AnsibleSequence'

I was slowly refactoring my standard playbooks to roles when I encountered this error. It took me a while to figure out what was wrong.

Start: the All-In-One playbook

myplaybook.yml
- hosts: rhel
  vars:
    ...
  tasks:
  - name: ...
    ...

This kind of playbook is hard to maintain. Also, some tasks where common to several playbooks, so I've decided to split it into several:

myplaybook.yml
- hosts: rhel
  vars:
    ...
  tasks:
  - name: ...
    include_tasks: path/to/task-1.yml
    include_tasks: path/to/task-2.yml
    ...

This was nice, but I encountered issues with variables that were not seen by sub-tasks. Must be mentioned that sub-tasks should not include the tasks keyword; only a list of tasks.

task-1.yml
  - name: ...
    ...

Here comes the roles

Roles has been precisely designed to split complex tasks into reusable modules.

playbooks
  roles
    my_custom_role
      defaults
        main.yml
      files
        my-cert-ad.pem
      handlers
        main.yml
      meta
        main.yml
      tasks
        main.yml
      templates
        ...
      vars
        main.yml

Ansible requires a standard directory structure; some of the names (in bold above) are like keywords. I mean: these folders don't have necessarily to exist, but if you create a folder named tasks or vars, the YAML scripts in these directories, they must follow some rules.

And this was precisely my mistake: a script inside the vars folder must not have the vars keyword, only a list of vars.

myvars.yml
vars:    <- remove this
  varname: "value"
  ...