diff --git a/src/Bundle/ChillPersonBundle/Form/PersonType.php b/src/Bundle/ChillPersonBundle/Form/PersonType.php
index eddf8c751..de9b3bbb1 100644
--- a/src/Bundle/ChillPersonBundle/Form/PersonType.php
+++ b/src/Bundle/ChillPersonBundle/Form/PersonType.php
@@ -114,10 +114,10 @@ class PersonType extends AbstractType
 
             $builder->get('placeOfBirth')->addModelTransformer(new CallbackTransformer(
                 static function ($string) {
-                    return strtoupper($string);
+                    return strtoupper((string) $string);
                 },
                 static function ($string) {
-                    return strtoupper($string);
+                    return strtoupper((string) $string);
                 }
             ));
         }