Notice for every submission without a file

With Webform 7.x-4.12, I get the following notice once for every form submission, that has no file attached:

Notice: ZipArchive::addFile(): Empty string as filename in webform_zip_exporter->post_process() (line 29 of sites/all/modules/contrib/webform_zip_exporter/includes/webform_zip_exporter.inc).

STR:
- Create a webform node with at least one optional file field.
- Submit the form without uploading a file.
- Export the results using webform_zip_exporter