Unable to save when using this widget within a field collection

Steps to reproduce:

1. Install Taxonomy single tag 7.x-1.x-dev (commit a9feede), Entity API and Field collection (One click simplytest.me link).
2. Navigate to /admin/structure/types/manage/article/fields, add a field collection field.
3. Navigate to /admin/structure/field-collections, Manage fields on the new field collection.
4. Add a term reference field to the field collection using the Single tag autocomplete text field widget.
5. Navigate to /node/add/article. Fill out the title and enter something in the term reference field in the field collection.

See these errors:

Notice: Undefined index: test tag in taxonomy_field_validate() (line 1530 of /home/dyb7/www/modules/taxonomy/taxonomy.module).
Notice: Trying to get property of non-object in taxonomy_field_validate() (line 1530 of /home/dyb7/www/modules/taxonomy/taxonomy.module).
Demo term reference field: illegal value.

Not sure if this is more of a field collection bug or more of a bug for this module, apologies if it's the former!