diff --git a/functions/_middleware.ts b/functions/_middleware.ts
index 6866b65..fcb3fd3 100644
--- a/functions/_middleware.ts
+++ b/functions/_middleware.ts
@@ -126,7 +126,7 @@ async function setTheme(context: RequestContext) {
   );
   const theme = themeCookie?.split("=").at(1);
 
-  if (!theme) context.data.theme = "dark";
+  if (!theme || !["dark", "light"].includes(theme)) context.data.theme = "dark";
   else context.data.theme = theme;
 
   return await context.next();