Unable to install theme: '…' due to unmet module dependencies: '1'

Question

I have a weird error with a custom theme (called uv_basic_theme). It ran well under Drupal 8, and now I've upgraded the site to Drupal 9. (9.4.4)

After upgrade, the status report listed this error:

1 (required)
UV Basic Theme requires this module.

Yes, it says "1 (required)". What could it mean?

I already assumed some faulty line in the info.yml, but I can't see anything wrong:

name: UV Basic Theme
description: Boilerplate Theme for Drupal 8
type: theme
base theme: true
core: 8.x
core_version_requirement: ^8 || ^9
libraries:
  - uv_basic_theme/global-css
  - uv_basic_theme/global-js
stylesheets-remove:
  - core/themes/stable/css/views/views.module.css
  - core/themes/stable/css/system/components/align.module.css
regions:
  header: 'Header'
  highlighted: 'Highlighted'
  navigation: 'Main Navigation'
  content: 'Main Content'
dependencies: []
module_dependencies: []

Yes, I've added dependencies and module_dependencies to see if it would fix it but no.

Answer 1

OK, I just saw the culprit:

base theme: true

Instead, it has to be either false, or a theme name.