The following error is thrown when trying to run drush en devel devel_generate -y in php 5.6.40:
Argument 1 passed to drush_chalk() must be an instance of string, string given,
called in /var/www/.drush/commands/pm/backdrop_pm_enable.drush.inc
on line 62 and defined output.inc:18 [error]
E_RECOVERABLE_ERROR encountered; aborting.
To ignore recoverable errors, run again with --no-halt-on-error [error]
Drush command terminated abnormally due to an unrecoverable error.
The same command works w/o throwing any errors in php 7.0+.