[gc2latex.pl] Invoice.pm: Can't call method "child"...

[cognitive.libertarian+ml at gmail.com]

Anyone using gc2latex to generate invoices as a LaTeX format?  I see
the following error when trying to generate an invoice using the stock
latex template that came with gc2latex:

  Can't call method "child" without a package or object reference at 
  /usr/share/perl5/Gnucash/Business/Invoice.pm line 32.

This is the code from line 32:

  my $id = $_->child("invoice:id")->value();