Warning: Invalid argument supplied for foreach() in _taxonomy_menu_trails_process()

I get the following error when navigate on a product page.
Some site peculiarities:
- site is multilanguage
- taxonomy menu trails is set on an product field which is a term reference with unlimited values
- I'm using a panel node template for the product content type
- this is a D6-->D7 experiment

Warning: Invalid argument supplied for foreach() in _taxonomy_menu_trails_process() (line 128 of ...\sites\all\modules\taxonomy_menu_trails\taxonomy_menu_trails.inc).