Undefined index: dropbox-files

Whenever we edit an item in our Omeka 2.2.2 site, the apache error files shows a line like this:

[Wed Mar 25 09:56:01 2015] [error] [client xxx.xxx.xxx.xxx] PHP Notice: Undefined index: dropbox-files in DropboxPlugin.php on line 89

At first i thought this was because there were a number of files in the /plugins/Dropbox/files directory. I removed those but the error message persists.

Omeka 2.2.2
PHP 5.4.35
Dropbox Plugin 0.7.1

here's line 89 of DropboxPlugin.php

$fileNames = $post['dropbox-files'];

appears within the hookAfterSaveItem() function which explains *when* I see the error but I don't know *why*