diff --git a/src/Plugin/search_api/processor/RenderedItem.php b/src/Plugin/search_api/processor/RenderedItem.php
index 10eaa15..0fbbb3a 100644
--- a/src/Plugin/search_api/processor/RenderedItem.php
+++ b/src/Plugin/search_api/processor/RenderedItem.php
@@ -211,7 +211,8 @@ class RenderedItem extends ProcessorPluginBase {
         continue;
       }
       $build = $item->getDatasource()->viewItem($item->getOriginalObject(), $this->configuration['view_mode'][$item->getDatasourceId()]);
-      $field->addValue($this->getRenderer()->renderPlain($build));
+      $content = $this->getRenderer()->renderPlain($build);
+      $field->addValue($content->__toString());
     }
 
     // Restore the original user.