Cant Send invoice Email error

[System Administrator - Andrew Wafula]

I have just upgraded from 1.2.3 to 1.3.1 and I get the following error when
trying to send invoices

Software error:

Can't send invoice email! at /usr/lib/perl5/site_perl/5.6.0/FS/cust_main.pm
line 837.

This is obtained after clicking 'Bill Now' on the Customer view page.

The following is what I have

RedHat 7.2
Apache 1.3.22
FS 1.3.1
Perl 5.6.1
MySQL 3.23.41
DBIx::DBSchema 0.19
Mail::Internet 1.43
Mail::Header 1.43

Please help,

Thanks,

Andrew