Taxonomy won't index after migration... invalid foreach in taxonomy.module line 1214

I'm in process of migrating a database to Drupal. The db is a custom hybrid of Wordpress and vBulletin.
I'm able to import the vb stuff into forums using vbtodrupal module. Also can import the Wordpress stuff using wordpress import module.
But after importing, I keep getting this error when I run cron
warning: Invalid argument supplied for foreach() in /home/folder/public_html/folder/modules/taxonomy/taxonomy.module on line 1214.
....over and over

Looked at line 1214 and best I can tell it's trying to index the taxonomy.

Anyway, here's what I'm wondering...
1. How serious is this (will it eventually kill the database?)
2. Ideas on how to clear it up?

If it helps... it seems like I only have the error when I do both the wordpress import and the vb import. Not either one by itself. (But I reached this conclusion about 2 AM so it may not be worth much!)

Also, if I delete the imported wordpress vocab, the error continues... even though it didn't start until after I imported wpress. (I do the vb first because most accts are there. Then the wrdpress because it can import user accts to existing accts).

Up against a deadline so any quick clues much appreciated!