Releasing Production Order *sometimes* gives me error -5002

I'm using 2007.

The way I'm releasing a production order is like this:

1. Call GetByKey()
2. Set ProductionOrderStatus = boposReleased
3. Call Update()

This usually works, but sometimes it produces error -5002 out of the blue, no clue why/what triggers this error.

Example of the error message is: The warehouse is not defined for the item.  [WOR1.wareHouse][line: 17] , 'Row no. 17'

However, after retry, this update will be successful.

Anybody know why, and how to resolve?

Thanks.