diff -u b/core/lib/Drupal/Core/Entity/ContentEntityForm.php b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
--- b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityForm.php
@@ -352,7 +352,7 @@
     // entity should be deeply serialized as part of the form object
     // serialization.
     foreach ($form_display->getComponents() as $component) {
-      if (($widget_definition = $this->widgetPluginManager->getDefinition($component['type'])) && isset($widget_definition['entity_deep_serialization']) && $widget_definition['entity_deep_serialization']) {
+      if (($widget_definition = $this->widgetPluginManager->getDefinition($component['type'])) && !empty($widget_definition['entity_deep_serialization'])) {
         $this->entityDeepSerializationRequired = TRUE;
         break;
       }