How to get tax table value for invoice report?

[Valdis Vītoliņš]

Hello!
I'm studying Bill/Invoice reports, and I investigate how for Fancy
invoice 
(fancy-invoice.scm) include field with Tax Table value (not simple T, if
it is taxed).

Currently I use
...
    (if (tax-col column-vector)
	(addto! row-contents
		(gnc:make-html-table-cell/markup
		 "number-cell"
		 (and (gncEntryGetBillTaxable entry) (gncEntryGetInvTaxTable
entry)))))
..
But it shows:
#[unknown] 
in report without any warnings/errors in the log.

Any ideas/directions (including API description, where I could look to
know how to invoke it) are welcomed.

Thanks in advance,
Valdis