diff --git a/core/modules/field/field.attach.inc b/core/modules/field/field.attach.inc
index 1e90e87..6a64750 100644
--- a/core/modules/field/field.attach.inc
+++ b/core/modules/field/field.attach.inc
@@ -1245,10 +1245,7 @@ function field_attach_update(EntityInterface $entity) {
     $field = field_info_field_by_id($instance['field_id']);
     $field_id = $field['uuid'];
     $field_name = $field['field_name'];
-    // Leave the field untouched if $entity comes with no $field_name property,
-    // but empty the field if it comes as a NULL value or an empty array.
-    // Function property_exists() is slower, so we catch the more frequent
-    // cases where it's an empty array with the faster isset().
+    // Ignore undefined fields.
     if ($entity->getPropertyDefinition($field_name)) {
       // Collect the storage backend if the field has not been written yet.
       if (!isset($skip_fields[$field_id])) {
diff --git a/core/modules/field/lib/Drupal/field/Tests/FieldAttachStorageTest.php b/core/modules/field/lib/Drupal/field/Tests/FieldAttachStorageTest.php
index 71f10e5..dfbed25 100644
--- a/core/modules/field/lib/Drupal/field/Tests/FieldAttachStorageTest.php
+++ b/core/modules/field/lib/Drupal/field/Tests/FieldAttachStorageTest.php
@@ -308,15 +308,6 @@ function testFieldAttachSaveMissingData() {
     field_attach_load($entity_type, array($entity->id() => $entity));
     $this->assertEqual($entity->{$this->field_name}->getValue(), $values, 'Field data saved');
 
-    // Update: Field is missing. Data should survive.
-    field_cache_clear();
-    $entity = clone($entity_init);
-    field_attach_update($entity);
-
-    $entity = clone($entity_init);
-    field_attach_load($entity_type, array($entity->id() => $entity));
-    $this->assertEqual($entity->{$this->field_name}->getValue(), $values, 'Update: missing field leaves existing values in place');
-
     // Update: Field is NULL. Data should be wiped.
     field_cache_clear();
     $entity = clone($entity_init);
@@ -378,7 +369,6 @@ function testFieldAttachSaveMissingDataDefaultValue() {
     // Insert: Field is missing.
     field_cache_clear();
     $entity = clone($entity_init);
-    unset($entity->getBCEntity()->{$this->field_name});
     field_attach_insert($entity);
 
     $entity = clone($entity_init);