Trying to turn on theme debug, but getting an error: Drush command terminated abnormally due to an unrecoverable error I'm new to Drupal and Drush and so am hoping someone can shed some light on this.

I tried to turn on Theme Debug with Drush:

drush vset theme_debug 1


But I keep getting an error I don't entirely understand:

Drush command terminated abnormally due to an unrecoverable error.                                             
Error: Declaration of Drush\Command\DrushInputAdapter::hasParameterOption() must be compatible with
Symfony\Component\Console\Input\InputInterface::hasParameterOption($values, $onlyParams = false) in
phar:///usr/local/bin/drush/lib/Drush/Command/DrushInputAdapter.php, line 27


I tried drush status too and got the same issue, so I think drush is probably broken across the board.

I tried deleting and reinstalling drush and reprovisioning my virtual box to no affect.

Would anyone know what could've gone wrong here?

(I am running Drush 8.1.7. on Drupal 7.5)
Update drush 

composer require drush/drush:9.0.0-alpha1