diff --git a/src/Normalizer/FieldItemListNormalizer.php b/src/Normalizer/FieldItemListNormalizer.php
index a719554..cb06725 100644
--- a/src/Normalizer/FieldItemListNormalizer.php
+++ b/src/Normalizer/FieldItemListNormalizer.php
@@ -16,7 +16,9 @@ use Symfony\Component\Serializer\Normalizer\SerializerAwareNormalizer;
  */
 class FieldItemListNormalizer extends SerializerAwareNormalizer implements NormalizerInterface {
 
-  const FORMAT = ['fbia', 'fbia_rss'];
+  const FORMAT_FBIA = 'fbia';
+
+  const FORMAT_FBIA_RSS  = 'fbia_rss';
 
   /**
    * Renderer service.
@@ -51,7 +53,7 @@ class FieldItemListNormalizer extends SerializerAwareNormalizer implements Norma
   public function supportsNormalization($data, $format = NULL) {
     // Only consider this normalizer if we are trying to normalize a field item
     // list into the 'fbia' format.
-    return in_array($format, static::FORMAT) && $data instanceof FieldItemListInterface;
+    return ($format === static::FORMAT_FBIA || $format === static::FORMAT_FBIA_RSS) && $data instanceof FieldItemListInterface;
   }
 
   /**