Mail::Message->study() limitations?

Are there any limitations on which headers one should study()?

This script breaks when the Subject is blank:

  #!/usr/bin/perl -w

  use strict;
  use Mail::Message;

  my $msg = Mail::Message->read(\*STDIN) or
	  die "unable to read message: $!\n";
  print $msg->study("subject"), "\n";

Stack trace:

Method produceBody() is not defined for a Mail::Message::Field::Unstructured.
Mail::Reporter::AUTOLOAD('Mail::Message::Fi...') called at
/usr/lib/perl5/site_perl/5.8.0/Mail/Message/Field/Full.pm line 142
Mail::Message::Field::Full::foldedBody('Mail::Message::Fi...') called
at /usr/lib/perl5/site_perl/5.8.0/Mail/Message/Field/Full.pm line 123
Mail::Message::Field::Full::unfoldedBody('Mail::Message::Fi...') called
at /usr/lib/perl5/site_perl/5.8.0/Mail/Message/Field/Full.pm line 162
Mail::Message::Field::Full::decodedBody('Mail::Message::Fi...') called
at /usr/lib/perl5/site_perl/5.8.0/Mail/Message/Field/Full.pm line 20

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