Not sure if this will be of any help but there is an outstanding bug with Tangram itself when using NULL DateTime types.... so perhaps related? See Tangram issue 26490 for more details.
This bug is still open and is over two years old. In fact Tangram & Class::Tangram haven't been updated on CPAN since 2006 (there is a development version 2.10_1 of Tangram from Jan 2007).
Still some good news the author/maintainer Sam Vilain does have the code on GitHub so you could chase him up over there.
Hope that helps a bit,
/I3az/
draegtun
2009-08-18 15:37:11