diff --git a/src/app/checkout/client.tsx b/src/app/checkout/client.tsx
index 3199a2e..0657ab7 100644
--- a/src/app/checkout/client.tsx
+++ b/src/app/checkout/client.tsx
@@ -56,7 +56,9 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
     setCheckoutError(null);
     try {
       // Mark that we're checking out so LogoutOnUnload doesn't clear cart
-      sessionStorage.setItem('isCheckingOut', 'true');
+      if (typeof window !== 'undefined') {
+        sessionStorage.setItem('isCheckingOut', 'true');
+      }
 
       const res = await fetch('/api/checkout', {
         method: 'POST',
@@ -73,7 +75,9 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
       });
       const data = await res.json();
       if (!res.ok || !data.url) {
-        sessionStorage.removeItem('isCheckingOut');
+        if (typeof window !== 'undefined') {
+          sessionStorage.removeItem('isCheckingOut');
+        }
         throw new Error(data.error ?? 'Something went wrong starting checkout.');
       }
       window.location.href = data.url;
@@ -122,7 +126,9 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
                   setCheckoutError(null);
                   try {
                     // Mark that we're checking out so LogoutOnUnload doesn't clear cart
-                    sessionStorage.setItem('isCheckingOut', 'true');
+                    if (typeof window !== 'undefined') {
+                      sessionStorage.setItem('isCheckingOut', 'true');
+                    }
 
                     const res = await fetch('/api/checkout', {
                       method: 'POST',
@@ -132,7 +138,9 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
                     const data = await res.json();
                     console.log('Checkout response:', { status: res.status, ok: res.ok, data });
                     if (!res.ok || !data.url) {
-                      sessionStorage.removeItem('isCheckingOut');
+                      if (typeof window !== 'undefined') {
+                        sessionStorage.removeItem('isCheckingOut');
+                      }
                       throw new Error(data.error ?? 'Something went wrong starting checkout.');
                     }
                     window.location.href = data.url;