diff --git a/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php b/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
index 9409aab0..3b58fd14 100644
--- a/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
+++ b/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
@@ -129,6 +129,9 @@ class FieldItemNormalizer extends NormalizerBase implements DenormalizerInterfac
     $data_internal = [];
     if (!empty($property_definitions)) {
       foreach ($data as $property_name => $property_value) {
+        if (!isset($property_definitions[$property_name])) {
+          continue;
+        }
         $property_value_class = $property_definitions[$property_name]->getClass();
         $data_internal[$property_name] = $denormalize_property($property_name, $property_value, $property_value_class, $format, $context);
       }