Composerizing Drupal 7 Project results in "Unable to find Drupal root directory"

[madamadami]

I've got an existing Drupal 7 project and want to composerize it. Folder .git as well as drupal root are located in "/var/www/html". To composerize, I cd into "/var/www/html" and then I launch the command composer composerize-drupal --composer-root=. --drupal-root=.
Unfortunately, this error message is thrown out:
"Unable to find Drupal root directory. Please change directories to a valid Drupal 8 application. Try specifying it with --drupal-root."
What's the issue here, is Drupal 7 not supported?