BUG? 'AnsibleSequence' object has no attribute 'strip'

I don't know if I have a syntax issue or a bug. Thought before filing a
bug I would see what you guys thought. I have a bunch of roles/playbooks I
wish to execute against new servers. Here is what I have come up with.
ansible-2.4.3.0-1.el7.ans.noarch

Playbook:
---
- hosts: "{{ host }}"

  roles:
    - ansible_dependencies
    - aliases

<snip />

  import_playbook:
    - /playbooks/one-offs/bash_history.yml -e host= "{{ host }}"
    - /playbooks/one-offs/dmesg_timestamps.yml -e host= "{{ host }}"
    - /playbooks/one-offs/grub_cmdline.yml -e host= "{{ host }}"

import_playbook is not a directive for a play, since it's a file with one or more play it's a directive on the top level.
So you need a dash in front and move it all the way to the left.

import_playbook doesn't take a list of playbook files, a little hint is that it's _playbook not _playbooks.

-e is misplaced, -e is a command line option so remove that.

It's not allowed to have space in-front or after the equal sign in Ansible yaml syntax.

So try

- import_playbook: /playbooks/one-offs/bash_history.yml host="{{ host }}"
- import_playbook: /playbooks/one-offs/dmesg_timestamps.yml host="{{ host }}"
...

Actually, you should write it like this:

- import_playbook: /playbooks/one-offs/bash_history.yml
  vars:
    host: "{{ host }}"

- import_playbook: /playbooks/one-offs/dmesg_timestamps.yml
  vars:
     host: "{{ host }}"