What could cause "Error Code 3349 - Cannot Convert UOM Back to Primary" to appear on Item Availabili

We use Item specific and Branch specific UOM. The error code refers to Business Function B4101220 - Calculate Availability.  We don't use secondary UOM's and the primary UOM of every item is EA. We have not set the UOM conversions back to Primary for all Units, but this is the same throughout the system and so far, this is the only item to cause this problem. When I look at the detailed availability for the branch with the error, Qty 1 On Hand is displayed, and no committed outbound items.

I have spot checked several other branches with the same item and there has been no error; likewise, I have checked this branch with several other items and there is no error.