012::-6531:ora-06531:Reference to uninitialized collection

Hello,

We are on 12.1.3 and have recently upgraded our database to 11g and since then seeing this new error very intermittently in Purchasing module when buyers respond to purchase order changes submitted by suppliers.The error that comes up is as follows:

012::-6531:ora-06531:Reference to uninitialized collection

We are unable to replicate the problem internally.Has anyone seen this error in oracle applications front end?

thanks

rahul