The function tries to add to the array $library['variants'] wich is not yet set.

<?php
$library
['variants'] += array($variant => array('installed' => FALSE));
?>

Comments

Status:Active» Postponed (maintainer needs more info)

Actually it is set by libraries_info_defaults() (see l. 355).
What is the setup in which you got that error?

Status:Postponed (maintainer needs more info)» Closed (cannot reproduce)

No response, marking fixed.

For reference, a similar issue: https://drupal.org/node/1988882

I received this error after a standard updating from 7.x-2.0 to 7.x-2.1 release, but the error line for me is 609.

Status:Closed (cannot reproduce)» Active

Status:Active» Closed (cannot reproduce)

After a little hacking, I got all caches to flush properly and now the error is gone. I got the idea from this issue https://drupal.org/node/1938638 and it seems to have helped. Maybe there's an issue with cached libraries and flushing caches upon update.

Putting ticket status back to what it was before I jumped in.