diff --git a/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php b/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
index 7df2a05..22c7e6e 100644
--- a/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
+++ b/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
@@ -16,6 +16,7 @@ use Drupal\jsonapi\JsonApiResource\JsonApiDocumentTopLevel;
 use Drupal\jsonapi\ResourceType\ResourceType;
 use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
 use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException;
 use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
 use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
@@ -143,7 +144,7 @@ class JsonApiDocumentTopLevelNormalizer extends NormalizerBase implements Denorm
         $canonical_ids = [];
         foreach ($id_list as $delta => $uuid) {
           if (empty($map[$uuid])) {
-            continue;
+            throw new NotFoundHttpException(sprintf('The resource identified by `%s:%s` (given as a relationship item) could not be found.', $relationship['data'][$delta]['type'], $uuid));
           }
           $reference_item = [
             'target_id' => $map[$uuid],
diff --git a/tests/src/Functional/NodeTest.php b/tests/src/Functional/NodeTest.php
index ca6a2bb..ed06941 100644
--- a/tests/src/Functional/NodeTest.php
+++ b/tests/src/Functional/NodeTest.php
@@ -355,7 +355,7 @@ class NodeTest extends ResourceTestBase {
 
     $random_uuid = \Drupal::service('uuid')->generate();
     $doc = $this->getPostDocument();
-    $doc['data']['relationships']['uid'] = [
+    $doc['data']['relationships']['uid']['data'] = [
       'type' => 'user--user',
       'id' => $random_uuid,
     ];
@@ -369,9 +369,23 @@ class NodeTest extends ResourceTestBase {
 
     // POST request: 404 when adding relationships to non-existing resources.
     $response = $this->request('POST', $url, $request_options);
-    // @todo Remove $expected + assertResourceResponse() in favor of the commented line below once https://www.drupal.org/project/jsonapi/issues/2943176 lands.
     $expected_document = [
-      '…',
+      'errors' => [
+        0 => [
+          'status' => 404,
+          'title' => 'Not Found',
+          'detail' => "The resource identified by `user--user:$random_uuid` (given as a relationship item) could not be found.",
+          'links' => [
+            'info' => [
+              'href' => 'http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.5',
+            ],
+            'via' => [
+              'href' => 'http://jsonapi.test/jsonapi/node/camelids',
+            ],
+          ],
+        ],
+      ],
+      'jsonapi' => static::$jsonApiMember,
     ];
     $this->assertResourceResponse(404, $expected_document, $response);
   }