plugin causes: Undefined variable: column_name in upload.php

Hello,

I wrote a plugin to upload files from the frontend. To all files uploded with the plugin i get the message:
Notice: Undefined variable: column_name in myPath\wp-admin\upload.php on line 439
On line 439 there is:
<?php if ( '0000-00-00 00:00:00' == $post->post_date && 'date' == $column_name )
This is the first time, that $column_name occours in the file upload.php, so I don’t know, where to start looking for the error.

Perhaps someone knows, where I have to look, or where the variable $column_name comes from.

Thanks Paka