diff --git a/src/Twig/AppRuntime.php b/src/Twig/AppRuntime.php
index 9269e88..e0cc9c0 100644
--- a/src/Twig/AppRuntime.php
+++ b/src/Twig/AppRuntime.php
@@ -95,8 +95,12 @@ class AppRuntime implements RuntimeExtensionInterface
         };
     }
 
-    public function mapGender(string $gender): string
+    public function mapGender(?string $gender): string
     {
+        if (null === $gender) {
+            return '';
+        }
+
         $gender = strtoupper($gender);
 
         return match ($gender) {