It looks like you are getting DBNull data for that PK value, try setting the Null Value Option in the BOM to Return Alternate on Null with zero or empty string based on your field type.
Glad to hear you found it. This type of things usually takes more than a couple of hours to find and most of the time it is under our nose .
I am sooooo glad that this happens to other programmers, too. Despite our best efforts to prove otherwise, we are all still quite human.
I feel your pain,Bill