Attempting to validate collections that aren't collections

validation.php

<?php

// Using Symfony 4

require("vendor/autoload.php");

use Symfony\Component\Validator\Validation;
use Symfony\Component\Validator\Constraints as Assert;

$goodData = array(
    "format" => array(
      "type" => "foo"
    )
  );

  $badData = array(
      "format" => array(
          "type" => "not foo or bar"
          )
      );

  $fatalData = array(
    "format" => "uh oh"
  );

  validate($goodData, "Good Data"); // 0 violations
  validate($badData, "Bad Data"); // 1 violation
  validate($fatalData, "Fatal Data"); // Fatal error

  function validate($data, $name)
  {
      $validator = Validation::createValidator(); // New validator interface
      $constraint = new Assert\Collection(array(
      "fields" => array(
        "format" => new Assert\Collection(array(
          "fields" => array(
            "type" => new Assert\Choice(["foo", "bar"])
          )
        ))
      )
    ));

      $violations = $validator->validate($data, $constraint); // Actually validate

      echo "Checked $name. Number of violations: " . count($violations) . "<br />";

      // Loop through each validation
      foreach ($violations as $v) {
          // Add the error to the error list
          echo $v->getMessage();
      }
  }