diff --git a/src/Bundle/ChillMainBundle/Serializer/Normalizer/PhonenumberNormalizer.php b/src/Bundle/ChillMainBundle/Serializer/Normalizer/PhonenumberNormalizer.php
index 7eb323754..f76b1e5c8 100644
--- a/src/Bundle/ChillMainBundle/Serializer/Normalizer/PhonenumberNormalizer.php
+++ b/src/Bundle/ChillMainBundle/Serializer/Normalizer/PhonenumberNormalizer.php
@@ -32,7 +32,7 @@ class PhonenumberNormalizer implements ContextAwareNormalizerInterface, Denormal
     }
 
     /**
-     * @param mixed $data
+     * @param string|null $data
      * @param mixed $type
      * @param null|mixed $format
      *
@@ -40,7 +40,7 @@ class PhonenumberNormalizer implements ContextAwareNormalizerInterface, Denormal
      */
     public function denormalize($data, $type, $format = null, array $context = [])
     {
-        if ('' === trim($data)) {
+        if ('' === trim((string) $data)) {
             return null;
         }