Parser error with INFO.YML file

I am attempting to create a Base Theme. I have all the necessary files, I have managed to create Themes that use others as a base theme (Classy, et.al.), and have not had this issue; however, when I attempt to move my new theme into my Drupal 8 setup, I get the famous "The website encountered an unexpected error. Please try again later." Error message. Checking the logs, I find
 

NOTICE: PHP message: Uncaught PHP Exception Drupal\Core\Extension\InfoParserException: "Unable to parse themes/custom/THEMENAME/THEMENAME.info.yml You cannot define a mapping item when in a sequence at line 5 (near "regions:")." at /var/www/html/docroot/core/lib/Drupal/Core/Extension/InfoParserDynamic.php line 53"

I am confused as to what this is saying, as I see nothing out of the ordinary in my INFO.YML file.

name: THEMENAME
type: theme
description: Theme for Drupal 8 & 9
package: core
base theme: false
core: 8.x
core_version_requirement: ^8 || ^9
version: 0.0.1

libraries:
  - tachometer/global-components

  regions:
    header: header
    search: search
    main_navigation: Main Navigation

    upper_block_1: Upper Block 1
    upper_block_2: Upper Block 2
    upper_block_3: Upper Block 3

I'm sure I'm missing something very obvious... I appreciate your help.