Invalid Item Number error when running Sales Update

I'm getting the following error when running sales update:

Subject       : Error: Item Number Invalid

CAUSE. . . . . The Item Number entered does not exist in the Basic Item Master

               (P4101).

RESOLUTION . . Enter a valid Item number or add the Item to the Basic Item

               Master (P4101).

I've checked the P4101 and it looks like the item number is set up correctly.  Also, if the item number was actually invalid, I would have expected it to give an error at the time the order was entered, right? 

Any ideas where to go from here? 

Thanks!