diff --git a/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php b/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
index cef3742ac1..8ab867e375 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php
@@ -376,8 +376,10 @@ protected function doSave($id, EntityInterface $entity) {
 
     $this->populateAffectedRevisionTranslations($entity);
 
-    // Populate the "revision_default" flag.
-    if ($this->entityType->isRevisionable()) {
+    // Populate the "revision_default" flag. We skip this when we are resaving
+    // the revision because this is only allowed for default revisions, and
+    // these cannot be made non-default.
+    if ($this->entityType->isRevisionable() && $entity->isNewRevision()) {
       $revision_default_key = $this->entityType->getRevisionMetadataKey('revision_default');
       $entity->set($revision_default_key, $entity->isDefaultRevision());
     }
diff --git a/core/tests/Drupal/KernelTests/Core/Entity/RevisionableContentEntityBaseTest.php b/core/tests/Drupal/KernelTests/Core/Entity/RevisionableContentEntityBaseTest.php
index b147f4ae4d..d478d19d32 100644
--- a/core/tests/Drupal/KernelTests/Core/Entity/RevisionableContentEntityBaseTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Entity/RevisionableContentEntityBaseTest.php
@@ -141,22 +141,39 @@ public function testWasDefaultRevision() {
     $entity = $storage->loadUnchanged($entity->id());
     $this->assertTrue($entity->wasDefaultRevision());
 
+    // Check that the "revision_default" flag cannot be changed once set.
+    /** @var \Drupal\entity_test_revlog\Entity\EntityTestMulWithRevisionLog $entity2 */
+    $entity2 = EntityTestMulWithRevisionLog::create([
+      'type' => $entity_type_id,
+    ]);
+    $entity2->save();
+    $this->assertTrue($entity2->wasDefaultRevision());
+    $entity2->isDefaultRevision(FALSE);
+    $entity2->save();
+    $this->assertTrue($entity2->wasDefaultRevision());
+
     // Simulate missing field data and verify an error is triggered in this
     // case, while the return value matches the entity object's default value
     // state.
+    /** @var \Drupal\entity_test_revlog\Entity\EntityTestMulWithRevisionLog $entity3 */
+    $entity3 = EntityTestMulWithRevisionLog::create([
+      'type' => $entity_type_id,
+    ]);
+    $entity3->save();
     /** @var \Drupal\Core\Database\Connection $database */
     $database = $this->container->get('database');
     $database->update($entity_type_id . '_revision')
       ->fields(['revision_default' => NULL])
+      ->condition('id', $entity3->id())
       ->execute();
-    $storage->resetCache([$entity->id()]);
-    $entity = $storage->load($entity->id());
-    $entity->isDefaultRevision(TRUE);
-    $this->assertTrue(@$entity->wasDefaultRevision());
-    $entity->isDefaultRevision(FALSE);
-    $this->assertFalse(@$entity->wasDefaultRevision());
+    $storage->resetCache([$entity3->id()]);
+    $entity3 = $storage->load($entity3->id());
+    $entity3->isDefaultRevision(TRUE);
+    $this->assertTrue(@$entity3->wasDefaultRevision());
+    $entity3->isDefaultRevision(FALSE);
+    $this->assertFalse(@$entity3->wasDefaultRevision());
     $this->setExpectedException('\PHPUnit_Framework_Error', "Missing data for the 'revision_default' field on the entity $entity_type_id:{$entity->id()}.");
-    $entity->wasDefaultRevision();
+    $entity3->wasDefaultRevision();
   }
 
   /**