diff --git a/back_end/src/controllers/webhooks.ts b/back_end/src/controllers/webhooks.ts
index 6919abf..2188dbf 100644
--- a/back_end/src/controllers/webhooks.ts
+++ b/back_end/src/controllers/webhooks.ts
@@ -18,12 +18,12 @@ export async function handleLNbitsPayment(req: Request, res: Response) {
     const webhookSecretQuery = (() => {
       const value = req.query?.secret;
       if (Array.isArray(value)) {
-        return value[0];
+        return typeof value[0] === 'string' ? value[0] : undefined;
       }
-      return value as string | undefined;
+      return typeof value === 'string' ? value : undefined;
     })();
 
-    const providedSecret = webhookSecretHeader || webhookSecretQuery || '';
+    const providedSecret: string = webhookSecretHeader || webhookSecretQuery || '';
 
     if (!lnbitsService.verifyWebhook(providedSecret)) {
       console.error('Webhook verification failed');