.../src/Functional/EntityResource/EntityResourceTestBase.php   | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 55825c8..fc712c9 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -1033,8 +1033,14 @@ public function testPatch() {
       // Change the rest_test_validation field to prove that then its validation
       // does run. In subsequent test assertions, it will not be modified, and
       // then should not trigger validation errors.
-      $modified_entity->get('rest_test_validation')->setValue('ALWAYS_FAIL');
-      $valid_request_body = $this->getNormalizedPatchEntity() + $this->serializer->normalize($modified_entity, static::$format);
+      $override = [
+        'rest_test_validation' => [
+          [
+            'value' => 'ALWAYS_FAIL',
+          ],
+        ],
+      ];
+      $valid_request_body = $override + $this->getNormalizedPatchEntity() + $this->serializer->normalize($modified_entity, static::$format);
       $request_options[RequestOptions::BODY] = $this->serializer->serialize($valid_request_body, static::$format);
       $response = $this->request('PATCH', $url, $request_options);
       $this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nrest_test_validation: REST test validation failed\n", $response);