Warning message when importing a journal article via DOI

I'm getting the following warning message when importing the following DOI (journal article): 10.1145/210181.210182

Notice: Undefined index: year in BiblioCrossRefClient->unixref_endElement() (line 194 of /var/www/html/drupal/istc-social/src/sites/all/modules/biblio/modules/crossref/biblio.crossref.client.php).

The resulting biblio node seems to be correct, however, and the "Year of Publication" field is getting properly set.

I've modified line 94 of biblio.crossref.client.php in order to remove the warning:
from
if (!isset($this->node['biblio_date'])) {
to
if (!isset($this->node['biblio_date']) && (!empty($this->node['year']))) {