Confusing error message when using --package-handler

While trying to change the package-handler when using pm-download, I got the following error message:

No  was set for including the package_handler engine curl. [0.17 sec, 7.62 MB]

(double space between "No" and "was").

The error message for "DRUSH_ENGINE INCLUDE_NO_PATH" includes $path, which does not make sense given that $path is empty when that error is thrown.