Bad registry entry for Sendgrid mailsystem

Error presents when attempting to save a new mail system with format type of HTMLmail via the mailsystem module.
( ! ) Fatal error: require_once(): Failed opening required '/Users/myuser/www/myproject.acquia/docroot//Users/myuser/www/myproject.acquia/files/mailsystem/HTMLMailSystem__SendGridMailSystem.mail.inc' (include_path='.:/usr/local/Cellar/php56/5.6.20/lib/php') in /Users/myuser/www/myproject/docroot/includes/bootstrap.inc on line 3186

Setting a conditional breakpoint at bootstrap.inc on line 3186 reveals that the path for the file is set like so:

if ($file) {
    require_once DRUPAL_ROOT . '/' . $file;
....

... but $file is already an absolute path.

Not sure if this is a sendgrid or a mailsystem bug.

Using the Registry Reload module clears the bug and then the site can be accessed.

drush rr

Not sure if this is a problem with mailsystem or sendgrid_integration. I'll update as I discover more, but it is my practice to report bugs as I am working on them.