Using PO_EXPRESS_CI to update PO line amounts to 0

We have a custom App Engine process where we call Component Interface PO_EXPRESS_CI to update the amounts on the PO Lines.  The business area using this custom process creates POs with lines that are used as placeholders for the PO amount (to keep track of the encumbrance).  As the actual items are purchased, the placeholder line amounts are reduced and new PO lines are added for the specific items.  Eventually these placeholder lines will be reduced to 0.  In the past, we would update PO_LINE_SHIP.PRICE_PO to 0, PO_LINE_SHIP.DISTRIB_MTHD_FLG to 'Q', and PO_LINE_DISTRIB.MERCHANDISE_AMT to 0 in our custom process.  However, this spring we ran PO rollover.  Now, whenever we are processing a PO line that has a Closed distribution line and an Open distribution line (partially vouchered at the time of the rollover), the CI fails with a divide by 0 error.  I'm wondering if we should be doing something different when these lines get to 0 to make this work.