diff --git a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
index 369e903..b3313dd 100644
--- a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
@@ -477,8 +477,20 @@ protected function postUpdateEntityTypeSchema(EntityTypeInterface $entity_type,
     // Rename the original tables so we can put them back in place in case
     // anything goes wrong.
     $backup_table_names = array_combine($original_table_mapping->getTableNames(), $backup_table_mapping->getTableNames());
-    foreach ($backup_table_names as $original_table_name => $backup_table_name) {
-      $this->database->schema()->renameTable($original_table_name, $backup_table_name);
+    $renamed_tables = [];
+    try {
+      foreach ($backup_table_names as $original_table_name => $backup_table_name) {
+        $this->database->schema()->renameTable($original_table_name, $backup_table_name);
+        $renamed_tables[$original_table_name] = $backup_table_name;
+      }
+    }
+    catch (\Exception $e) {
+      foreach ($renamed_tables as $original_table_name => $backup_table_name) {
+        $this->database->schema()->renameTable($backup_table_name, $original_table_name);
+      }
+
+      // Re-throw the original exception.
+      throw $e;
     }
 
     // Put the new tables in place and update the entity type and field storage
diff --git a/core/lib/Drupal/Core/Entity/Sql/SqlEntityTypeSchemaListenerTrait.php b/core/lib/Drupal/Core/Entity/Sql/SqlEntityTypeSchemaListenerTrait.php
index d6f840b..b019670 100644
--- a/core/lib/Drupal/Core/Entity/Sql/SqlEntityTypeSchemaListenerTrait.php
+++ b/core/lib/Drupal/Core/Entity/Sql/SqlEntityTypeSchemaListenerTrait.php
@@ -220,15 +220,15 @@ protected function copyData(EntityTypeInterface $entity_type, EntityTypeInterfac
       $sandbox['current_id'] = $identifier;
     }
 
-    // If we're not in maintenance mode, the number of entities could change at
-    // any time so make sure that we always use the latest record count.
+    // Get an updated count of entities that still need to migrated to the new
+    // storage.
     $missing = $this->database->select($table_name, 't')
       ->condition("t.$identifier_field", $sandbox['current_id'], '>')
       ->orderBy($identifier_field, 'ASC')
       ->countQuery()
       ->execute()
       ->fetchField();
-    $sandbox['#finished'] = $sandbox['progress'] / ($sandbox['progress'] + (int) $missing);
+    $sandbox['#finished'] = $missing ? $sandbox['progress'] / ($sandbox['progress'] + (int) $missing) : 1;
   }
 
   /**