diff --git a/src/Normalizer/JsonBlueprintDenormalizer.php b/src/Normalizer/JsonBlueprintDenormalizer.php
index c6330ffea6d420b30d98b8b9e8cce95172af35f8..b945ba59a8f3103549975eef10cc28e4d179ca68 100644
--- a/src/Normalizer/JsonBlueprintDenormalizer.php
+++ b/src/Normalizer/JsonBlueprintDenormalizer.php
@@ -31,10 +31,30 @@ class JsonBlueprintDenormalizer implements DenormalizerInterface, SerializerAwar
    */
   protected $logger;
 
+  /**
+   * The schema validator.
+   *
+   * This property will only be set if the validator library is available.
+   *
+   * @var \JsonSchema\Validator|null
+   */
+  protected $validator;
+
   public function __construct(LoggerInterface $logger) {
     $this->logger = $logger;
   }
 
+  /**
+   * Sets the validator service if available.
+   */
+  public function setValidator(Validator $validator = NULL) {
+    if ($validator) {
+      $this->validator = $validator;
+    }
+    elseif (class_exists(Validator::class)) {
+      $this->validator = new Validator();
+    }
+  }
 
   /**
    * {@inheritdoc}
@@ -50,10 +70,7 @@ class JsonBlueprintDenormalizer implements DenormalizerInterface, SerializerAwar
    * {@inheritdoc}
    */
   public function denormalize($data, $class, $format = NULL, array $context = []) {
-    assert(
-      '$this->validateInput($data)',
-      'The input blueprint failed validation (see the logs for details). Please report this in the issue queue on drupal.org'
-    );
+    $this->doValidateInput($data);
     $data = array_map([$this, 'fillDefaults'], $data);
     $subrequests = array_map(function ($item) {
       return new Subrequest($item);
@@ -138,6 +155,18 @@ class JsonBlueprintDenormalizer implements DenormalizerInterface, SerializerAwar
     return $raw_item;
   }
 
+
+  /**
+   * Wraps validation in an assert to prevent execution in production.
+   *
+   * @see self::validateInput
+   */
+  public function doValidateInput($input) {
+    if (PHP_MAJOR_VERSION >= 7 || assert_options(ASSERT_ACTIVE)) {
+      assert($this->validateInput($input), 'A Subrequests blueprint failed validation (see the logs for details). Please report this in the issue queue on drupal.org');
+    }
+  }
+
   /**
    * Validates a response against the JSON API specification.
    *
@@ -148,26 +177,26 @@ class JsonBlueprintDenormalizer implements DenormalizerInterface, SerializerAwar
    *   FALSE if the input failed validation, otherwise TRUE.
    */
   protected function validateInput($input) {
-    if (!class_exists("\\JsonSchema\\Validator")) {
+    // If the validator isn't set, then the validation library is not installed.
+    if (!$this->validator) {
       return TRUE;
     }
 
-    $validator = new Validator();
     $schema_path = dirname(dirname(__DIR__)) . '/schema.json';
 
-    $validator->check($input, (object) ['$ref' => 'file://' . $schema_path]);
+    $this->validator->check($input, (object) ['$ref' => 'file://' . $schema_path]);
 
-    if (!$validator->isValid()) {
+    if (!$this->validator->isValid()) {
       // Log any potential errors.
       $this->logger->debug('Response failed validation: @data', [
         '@data' => Json::encode($input),
       ]);
       $this->logger->debug('Validation errors: @errors', [
-        '@errors' => Json::encode($validator->getErrors()),
+        '@errors' => Json::encode($this->validator->getErrors()),
       ]);
     }
 
-    return $validator->isValid();
+    return $this->validator->isValid();
   }
 
   /**
diff --git a/subrequests.services.yml b/subrequests.services.yml
index 1c6bd7da0e740b0b6be84e8f9ea2c4045fe4ec79..0185fe81bbafac11c99e357745151af6ca9c62c1 100644
--- a/subrequests.services.yml
+++ b/subrequests.services.yml
@@ -15,6 +15,8 @@ services:
     arguments: ['@logger.channel.subrequests']
     tags:
       - { name: normalizer, priority: 0 }
+    calls:
+      - [setValidator, []]
   subrequests.denormalizer.subrequest.json:
     class: Drupal\subrequests\Normalizer\JsonSubrequestDenormalizer
     tags:
diff --git a/tests/src/Unit/SubrequestsManagerTest.php b/tests/src/Unit/SubrequestsManagerTest.php
index 2d2912c4eb45e3002908dabce095ec8c78b59a85..c48d5d57866be7b69b90c2b6129c9f340c125e6a 100644
--- a/tests/src/Unit/SubrequestsManagerTest.php
+++ b/tests/src/Unit/SubrequestsManagerTest.php
@@ -89,6 +89,7 @@ class SubrequestsManagerTest extends UnitTestCase {
     ]);
     $tree->stack([$subrequests[0]]);
     $tree->stack([$subrequests[1], $subrequests[2]]);
+    $tree->setMasterRequest(new Request());
     $actual = $this->sut->request($tree);
     $this->assertSame('<foo>', $actual[0]->headers->get('Content-ID'));
     $this->assertSame('<oop>', $actual[1]->headers->get('Content-ID'));