> db.test.update({}, { $set : {} });
WriteResult({
"nMatched" : 0,
"nUpserted" : 0,
"nModified" : 0,
"writeError" : {
"code" : 9,
"errmsg" : "'$set' is empty. You must specify a field like so: {$mod: {<field>: ...}}"
}
})

Ideally should output

"'$set' is empty. You must specify a field like so: {$set: {<field>: ...}}"

instead.