Error when receiving on a Purchase Order

Hi there - When receiving on a Purchase Order (not created from a requisition) we sometimes get the following error (see attached screen shot). We've noticed that some POs resulting in this error are due to $0 line amount however, other POs have dollar amounts on the line being received. Further research shows that the PO in general is reserved and approved however, the specific lines resulting in this error don't appear to be encumbered (encumbered_flag = 'N'). We can't reproduce this at will and wondering if anyone else has seen this or may have any thoughts on what is causing this. Also, we don't