diff -u b/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php b/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
--- b/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
+++ b/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
@@ -281,7 +281,7 @@
   /**
    * Performs mimimal validation of the document.
    */
-  public function validateRequestBody($document) {
+  protected static function validateRequestBody(array $document) {
     // Ensure that the relationships key was not placed in the top level.
     if (isset($document['relationships']) && !empty($document['relationships'])) {
       throw new BadRequestHttpException("Found \"relationships\" within the document's top level. The \"relationships\" key must be within resource object.");
diff -u b/tests/src/Functional/JsonApiFunctionalTest.php b/tests/src/Functional/JsonApiFunctionalTest.php
--- b/tests/src/Functional/JsonApiFunctionalTest.php
+++ b/tests/src/Functional/JsonApiFunctionalTest.php
@@ -594,10 +594,10 @@
       ],
     ]);
     $created_response = Json::decode((string) $response->getBody());
-    $this->assertEquals(400, $response->getStatusCode());
+    $this->assertSame(400, $response->getStatusCode());
     $this->assertNotEmpty($created_response['errors']);
-    $this->assertEquals("Bad Request", $created_response['errors'][0]['title']);
-    $this->assertEquals("Found \"relationships\" within the document's top level. The \"relationships\" key must be within resource object.", $created_response['errors'][0]['detail']);
+    $this->assertSame("Bad Request", $created_response['errors'][0]['title']);
+    $this->assertSame("Found \"relationships\" within the document's top level. The \"relationships\" key must be within resource object.", $created_response['errors'][0]['detail']);
     // 6.2 "type" not included in "data"
     $missing_type = $body;
     unset($missing_type['data']['type']);
@@ -610,10 +610,10 @@
       ],
     ]);
     $created_response = Json::decode((string) $response->getBody());
-    $this->assertEquals(400, $response->getStatusCode());
+    $this->assertSame(400, $response->getStatusCode());
     $this->assertNotEmpty($created_response['errors']);
-    $this->assertEquals("Bad Request", $created_response['errors'][0]['title']);
-    $this->assertEquals("Resource object must include a \"type\".", $created_response['errors'][0]['detail']);
+    $this->assertSame("Bad Request", $created_response['errors'][0]['title']);
+    $this->assertSame("Resource object must include a \"type\".", $created_response['errors'][0]['detail']);
     // 7. Successful PATCH.
     $body = [
       'data' => [