> db.exists.insert({})
WriteResult({ "nInserted" : 1 })
> db.exists.runCommand('delete', {deletes: [{q:{$invalid: true}, limit:0}]})
{
        "ok" : 1,
        "n" : 0,
        "writeErrors" : [
                {
                        "index" : 0,
                        "code" : 17218,
                        "errmsg" : "Can't canonicalize query { $invalid: true }"
                }
        ]
}
> db.doesnt_exist.runCommand('delete', {deletes: [{q:{$invalid: true}, limit:0}]})
{ "ok" : 1, "n" : 0 }