Theme Editor: Backup file messages inconsistent

I created a backup directory myself, and probably gave it insufficient permissions (766), so that when I saved a stylesheet, style.css with the option "Save original to the backup file", I received a warning that it was "Unable to create backup" and a message that the "Backup was created". In full:

• warning: copy(sites/all/themes/future/theme_editor_backup/style.css.bak) [function.copy]: failed to open stream: Permission denied in /home//www.mydomain.com/public_html/sites/all/modules/theme_editor/theme_editor... on line 578.
• Unable to create backup of file (sites/all/themes/future/style.css)

• Backup was created
• File changes written to file (sites/all/themes/future/style.css)
• File was saved successfully

The backup file was not created.