Cannot unset string offsets

Some of my users have been getting this error:

Fatal error: Cannot unset string offsets in .../sites/all/modules/authcache/authcache.module on line 560

Line 559-561 are these lines in the authcache.module code
if ($user->uid && $is_page_authcache) {
unset($links['comment_edit']);
}

If you change it to this, it seems to work for all users:
if ($user->uid && $is_page_authcache && isset($links['comment_edit'])) {
unset($links['comment_edit']);
}