original line:
if($wbibmacros[$field_text] != ""){
changed line:
if(isset($wbibmacros[$field_text]) != ""){
The original line causes a lot of warning entries in the error logs, such as
PHP Notice: Undefined offset: 2011 in .../extensions/BibTex/BibTex.php on line 353
PHP Notice: Undefined index: Pittsburgh, PA in .../extensions/BibTex/BibTex.php on line 353