Undefined index: mongodb foreach php

I'm testing out mongodb with php and I have run in to a problem I don't know how to solve, this is the code I am using at the moment.

<?php

require ($_SERVER['DOCUMENT_ROOT'] . '/grafic/mongodb_php/vendor/autoload.php'); 

$client = new MongoDB\Client;
$snrdb = $client->snrdb;
$nodos_live = $snrdb->nodos_live;

$filter  = [];
$options = ['sort' => ['_id' => 1], 'limit' => 1];

$show = $nodos_live->find($filter, $options);

foreach ($show as $collection) {
    $snr_1= $collection["n1_snr_1"];
    $snr_2= $collection["n1_snr_2"];
}

echo $snr_1 ." ";
echo $snr_2;

?>

this gives me

28.9 28.3

now if I change the id and collection name to 2 and n2_snr_* like so.

<?php

require ($_SERVER['DOCUMENT_ROOT'] . '/grafic/mongodb_php/vendor/autoload.php'); 

$client = new MongoDB\Client;
$snrdb = $client->snrdb;
$nodos_live = $snrdb->nodos_live;

$filter  = [];
$options = ['sort' => ['_id' => 2], 'limit' => 1];

$show = $nodos_live->find($filter, $options);

foreach ($show as $collection) {
    $snr_1= $collection["n2_snr_1"];
    $snr_2= $collection["n2_snr_2"];
}

echo $snr_1 ." ";
echo $snr_2;

?>

this gives me this ERROR msg:

Fatal error: Uncaught MongoDB\Driver\Exception\ConnectionException: bad sort specification in C:\xampp\htdocs\grafic\mongodb_php\vendor\mongodb\mongodb\src\Operation\Find.php:287 Stack trace: #0 C:\xampp\htdocs\grafic\mongodb_php\vendor\mongodb\mongodb\src\Operation\Find.php(287): MongoDB\Driver\Server->executeQuery('snrdb.nodos_liv...', Object(MongoDB\Driver\Query), Array) #1 C:\xampp\htdocs\grafic\mongodb_php\vendor\mongodb\mongodb\src\Collection.php(531): MongoDB\Operation\Find->execute(Object(MongoDB\Driver\Server)) #2 C:\xampp\htdocs\grafic\test\test.php(23): MongoDB\Collection->find(Array, Array) #3 {main} thrown in C:\xampp\htdocs\grafic\mongodb_php\vendor\mongodb\mongodb\src\Operation\Find.php on line 287

And I can't understand why I get this ERROR msg anyone that can help me with this?

If a do a dump like so

<?php 

require ($_SERVER['DOCUMENT_ROOT'] . '/grafic/mongodb_php/vendor/autoload.php'); 

$client = new MongoDB\Client;
$snrdb = $client->snrdb;
$nodos_live = $snrdb->nodos_live;

$show = $nodos_live->findOne(
    ['_id' => '1']
);

var_dump($show)

?>

I get this showing that ther is a document with id 1

object(MongoDB\Model\BSONDocument)#19 (1) {
  ["storage":"ArrayObject":private]=>
  array(5) {
    ["_id"]=>
    string(1) "1"
    ["n1_snr_1"]=>
    float(28.9)
    ["n1_snr_2"]=>
    float(28.3)
    ["n1_snr_3"]=>
    string(1) "0"
    ["time_stamp"]=>
    string(19) "2018-05-31 21:44:22"
  }
}

And same thig I i do a dump of _id => 2 it shows that it exist as well.

object(MongoDB\Model\BSONDocument)#19 (1) {
  ["storage":"ArrayObject":private]=>
  array(5) {
    ["_id"]=>
    string(1) "2"
    ["n2_snr_1"]=>
    float(31.7)
    ["n2_snr_2"]=>
    float(35.7)
    ["n2_snr_3"]=>
    string(1) "0"
    ["time_stamp"]=>
    string(19) "2018-05-31 21:44:22"
  }
}