diff --git a/app/(chat)/api/billing/checkout/route.ts b/app/(chat)/api/billing/checkout/route.ts
index f518240..8cb3c46 100644
--- a/app/(chat)/api/billing/checkout/route.ts
+++ b/app/(chat)/api/billing/checkout/route.ts
@@ -26,10 +26,14 @@ export async function POST(request: Request) {
   const baseUrl =
     process.env.NEXT_PUBLIC_BASE_URL ?? new URL(request.url).origin;
 
+  const rawEmail = session.user.email ?? "";
+  const looksLikeEmail = /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(rawEmail);
+  const customerEmail = looksLikeEmail ? rawEmail : undefined;
+
   try {
     const result = await createCheckoutSession({
       userId: session.user.id,
-      customerEmail: session.user.email ?? undefined,
+      customerEmail,
       appSlug,
       baseUrl,
     });