Drush css-js cache clear error There are times when I run drush cc css-js and I get the following error: 

'css-js' cache is not a valid cache type   

Does anyone know why this may happen?

Thanks,
The latest dev releases of Drush (both 8.x-6.x and 7.x-5.x) now provide a better error message when attempting to select a specific cache type (e.g. css-js) without first selecting a working Drupal site.  c.f. http://drupal.org/node/1954552

Note also that when selecting a Drupal site, cd /path/to/drupal/root only works if settings.php is stored in a directory called "default".  Otherwise, you will need to instead cd /path/to/drupal/root/sites/mysite.com before using drush cc css-js.  You can also use the --root and --uri parameters, e.g. drush --root=/path/to/drupal/root --uri=http://mysite.com cc css-js