Engine variable set to NULL on settings page

On the admin/config/system/mimemail settings page there is a variable_set in the form constructor. If you have only one Mimemail engine, it doesn't display the select, and auto-sets to that engine on the form constructor call. But on my environment it sets the variable as NULL rather than "mimemail". That's because foreach() moves the internal pointer and than key() returns NULL.

Environment: PHP Version 5.2.10-2ubuntu6.5