diff --git a/src/runtime/bake/DevServer.rs b/src/runtime/bake/DevServer.rs
index a765e21ba6cb..6c3156d2eb4b 100644
--- a/src/runtime/bake/DevServer.rs
+++ b/src/runtime/bake/DevServer.rs
@@ -1737,13 +1737,14 @@ fn on_js_request(dev: &mut DevServer, req: &mut Request, resp: AnyResponse) {
         return;
     }
 
-    let route_bundle_index =
-        route_bundle::Index::init(u32::try_from(id & 0xFFFFFFFF).expect("int cast"));
-    let generation: u32 = u32::try_from(id >> 32).expect("int cast");
+    let index = id as u32;
+    let generation = (id >> 32) as u32;
 
-    if route_bundle_index.get() as usize >= dev.route_bundles.len() {
+    // The URL can hold any u32 and `Index::init` asserts on `u32::MAX`: check the range first.
+    if index as usize >= dev.route_bundles.len() {
         return not_found(resp);
     }
+    let route_bundle_index = route_bundle::Index::init(index);
 
     let route_bundle = &dev.route_bundles[route_bundle_index.get() as usize];
     if route_bundle.client_script_generation != generation
diff --git a/test/bake/dev/html.test.ts b/test/bake/dev/html.test.ts
index 978cfedcc886..f7cf08d3989c 100644
--- a/test/bake/dev/html.test.ts
+++ b/test/bake/dev/html.test.ts
@@ -373,6 +373,32 @@ devTest("error report endpoint blanks stray non-text bytes in reported frames",
     await dev.fetch("/").expect.toInclude("<h1>Frame Bytes</h1>");
   },
 });
+
+devTest("client script route answers 404 when the route bundle index is out of range", {
+  files: {
+    "index.html": emptyHtmlFile({
+      scripts: ["/script.ts"],
+      body: "<h1>Client Script</h1>",
+    }),
+    "script.ts": `
+      console.log("client-script-marker");
+    `,
+  },
+  async test(dev) {
+    // `/_bun/client/{name}-{index}{generation}.js`: each hex group holds the four bytes of a u32.
+    const page = await dev.fetch("/").text();
+    const [script, generation] = page.match(/\/_bun\/client\/index-[0-9a-f]{8}([0-9a-f]{8})\.js/) ?? [];
+    expect(script).toBeString();
+    await dev.fetch(script).expect.toInclude("client-script-marker");
+
+    // `ffffffff` is the value that the index type reserves for "no index".
+    await dev.fetch(`/_bun/client/index-ffffffff${generation}.js`).expect404();
+    await dev.fetch(`/_bun/client/index-feffffff${generation}.js`).expect404();
+
+    await dev.fetch("/").expect.toInclude("<h1>Client Script</h1>");
+  },
+});
+
 devTest("editing a file imported from outside the project root hot-reloads", {
   // The Windows watcher does not watch files outside the project directory.
   skip: ["win32"],