What's wrong with this code?

Okay guys, I need another pair of eyes to try find an error in my code.
Can any of you see anything wrong with this bit of code?

$mesg = $entry->add('objectClass' => 'ntUser',
		'ntUserDomainId' => $ntUserDomainId,
		'ntUserHomeDir' => "\\\\host.tld\\home\\$ntUserDomainId"
		);

.

When I run my program, I get this error:

Use of uninitialized value in concatenation (.) or string at
./add_nt_attribs.pl line 120, <STDIN> line 1.
Use of uninitialized value in length at
/usr/local/perl-5.8/lib/site_perl/5.8.8/Convert/ASN1/_encode.pm line
143, <STDIN> line 1.
Use of uninitialized value in concatenation (.) or string at
/usr/local/perl-5.8/lib/site_perl/5.8.8/Convert/ASN1/_encode.pm line
144, <STDIN> line 1.
ntUserDomainId: value #0 invalid per syntax at
/usr/local/perl-5.8/lib/site_perl/5.8.8/Net/LDAP/Entry.pm line 268

$entry already exists in LDAP, and I'm just trying to add this new
objectClass and related attributes to the entry. Am I doing this the
wrong way?

Should I be using a function other than entry->add(), like $ldap->modify()?

-- 
Prentice