.../src/Plugin/rest/resource/EntityResource.php    |  1 +
 .../EntityResource/EntityResourceTestBase.php      | 28 ++++++++++------------
 2 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
index b356ca6..dbd9a09 100644
--- a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
+++ b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
@@ -226,6 +226,7 @@ public function patch(EntityInterface $original_entity, EntityInterface $entity
     }
 
     // Overwrite the received fields.
+    // @todo Remove $changed_fields in https://www.drupal.org/node/2906129.
     $changed_fields = [];
     foreach ($entity->_restSubmittedFields as $field_name) {
       $field = $entity->get($field_name);
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 07b04ef..1fc1f46 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -1092,22 +1092,20 @@ public function testPatch() {
         $modified_entity->get($patch_protected_field_name)->setValue($original_values[$patch_protected_field_name]);
       }
 
-      if ($this->entity->hasField('rest_test_validation')) {
-        // 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);
-        $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);
+      // 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);
+      $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);
 
-        // Set the rest_test_validation field to always fail validation, which
-        // allows asserting that not modifying that field does not trigger
-        // validation errors.
-        $this->entity->set('rest_test_validation', 'ALWAYS_FAIL');
-        $this->entity->save();
-      }
+      // Set the rest_test_validation field to always fail validation, which
+      // allows asserting that not modifying that field does not trigger
+      // validation errors.
+      $this->entity->set('rest_test_validation', 'ALWAYS_FAIL');
+      $this->entity->save();
     }
 
     // 200 for well-formed PATCH request that sends all fields (even including