Is there any way to get a Condition to ignore the fact that a field isn't on a Record? I have a WF running on Vendor Bills and Purchase Orders, and the fields vary slightly for my conditions. I basically have a Condition on a State like:

(
  fieldA changed OR
  fieldB changed OR
  fieldC changed OR
  fieldD changed
)

where

fieldC

is only on POs and

fieldD

is only on VBs