db.collection.update() upsert behavior change from 2.6 when query includes predicate on _id

Given the following query and update objects: q={ "_id.hash" : "e6577bc7" } u={ "count" : 43.97} If we run update, no upsert, no matches, no updates as expected: > db.coll_name.update(q,u) WriteResult({ "nMatched" : 0, "nUpserted" : 0, "nModified" : 0 }) If we make it an upsert it will fail (this is not the case in 2.6) > db.coll_name.update(q,u,upsert=true) WriteResult({ "nMatched" : 0, "nUpserted" : 0, "nModified" : 0, "writeError" : { "code" : 111, "errmsg" : "field at '_id' must be exactly specified, field at sub-path '_id.hash'found" } }) If we have a match for the query, it will succeed. Only the upsert behavior has changed. q={ "_id.hash" : "e6577bc37" } u={ "count" : 49.97} > db.coll_name.update(q,u,upsert=true) WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })