Warning: Invalid argument supplied for foreach() in form_type_checkboxes_value() (line 2302 of includes/form.inc).

Problem/Motivation

Whenever I go to a Nodequeue edit page like:
http://example.com/admin/structure/nodequeue/81/view/

I receive the error:

Warning: Invalid argument supplied for foreach() in form_type_checkboxes_value() (line 2302 of includes/form.inc).

Proposed resolution

Ensure the form values have the correct variable type.