diff --git a/src/Normalizer/TextItemNormalizer.php b/src/Normalizer/TextItemNormalizer.php
index 4d751f6..3dd072b 100644
--- a/src/Normalizer/TextItemNormalizer.php
+++ b/src/Normalizer/TextItemNormalizer.php
@@ -16,6 +16,11 @@ class TextItemNormalizer extends FieldItemNormalizer {
    */
   protected $supportedInterfaceOrClass = 'Drupal\text\Plugin\Field\FieldType\TextItemBase';
 
+  /**
+   * @var string[]
+   */
+  protected $format = ['json'];
+
   /**
    * {@inheritdoc}
    */