diff --git a/backend/src/middleware/validators.js b/backend/src/middleware/validators.js
index 884fdfd..bc951aa 100644
--- a/backend/src/middleware/validators.js
+++ b/backend/src/middleware/validators.js
@@ -70,9 +70,10 @@ const registerValidation = [
     .matches(/^[a-zA-ZÀ-ÿ\s'-]+$/)
     .withMessage('Last name contains invalid characters'),
   body('wsdcId')
-    .optional()
-    .trim()
-    .matches(/^\d{1,10}$/)
+    .optional({ nullable: true, checkFalsy: true })
+    .isLength({ min: 1, max: 10 })
+    .withMessage('WSDC ID must be between 1 and 10 characters')
+    .matches(/^\d+$/)
     .withMessage('WSDC ID must be numeric (max 10 digits)'),
   // Turnstile CAPTCHA (only required if TURNSTILE_SECRET_KEY is set)
   ...(process.env.TURNSTILE_SECRET_KEY