diff --git a/src/app/EspController.ts b/src/app/EspController.ts
index 81e1711..fd1ab2c 100644
--- a/src/app/EspController.ts
+++ b/src/app/EspController.ts
@@ -57,7 +57,9 @@ export default class EspController {
     ) => void,
   ) {
     if (data.length !== 0x1000000) {
-      throw `Data length must be 0x1000000, but got 0x${data.length.toString().padStart(7, "0")}`;
+      throw new Error(
+        `Data length must be 0x1000000, but got 0x${data.length.toString().padStart(7, "0")}`,
+      );
     }
 
     await this.writeData(data, 0, reportProgress);
@@ -72,7 +74,9 @@ export default class EspController {
     ) => void,
   ) {
     if (data.length !== 0x2000) {
-      throw `Data length must be 0x2000, but got 0x${data.length.toString().padStart(4, "0")}`;
+      throw new Error(
+        `Data length must be 0x2000, but got 0x${data.length.toString().padStart(4, "0")}`,
+      );
     }
 
     await this.writeData(data, 0xe000, reportProgress);
@@ -102,10 +106,12 @@ export default class EspController {
     ) => void,
   ) {
     if (data.length > 0x640000) {
-      throw `Data cannot be larger than 0x640000`;
+      throw new Error(`Data cannot be larger than 0x640000`);
     }
     if (data.length < 0x320000) {
-      throw `Data seems too small, are you sure this is the right file?`;
+      throw new Error(
+        `Data seems too small, are you sure this is the right file?`,
+      );
     }
 
     await this.writeData(data, 0x10000, reportProgress);
diff --git a/src/app/firmwareFetcher.ts b/src/app/firmwareFetcher.ts
new file mode 100644
index 0000000..a196a92
--- /dev/null
+++ b/src/app/firmwareFetcher.ts
@@ -0,0 +1,17 @@
+"use server";
+
+const urls: Record<string, string> = {
+  "3.0.8":
+    "http://gotaserver.xteink.com/api/download/ESP32C3/V3.0.8/V3.0.8-EN.bin",
+};
+
+export async function getFirmware(version: string) {
+  const url = urls[version];
+
+  if (!url) {
+    throw new Error("Unknown firmware version");
+  }
+
+  const response = await fetch(url);
+  return new Uint8Array(await response.arrayBuffer());
+}
diff --git a/src/app/page.tsx b/src/app/page.tsx
index bb1b1b5..088d6c0 100644
--- a/src/app/page.tsx
+++ b/src/app/page.tsx
@@ -5,13 +5,7 @@ import EspController from "@/app/EspController";
 import Step from "@/components/Step/Step";
 
 import styles from "./page.module.css";
-
-const downloadEnglishFirmware = async () => {
-  const a = await fetch(
-    "http://gotaserver.xteink.com/api/download/ESP32C3/V3.0.8/V3.0.8-EN.bin",
-  );
-  return new Uint8Array(await a.arrayBuffer());
-};
+import { getFirmware } from "@/app/firmwareFetcher";
 
 const downloadData = (data: Uint8Array, fileName: string, mimeType: string) => {
   // @ts-expect-error types say no, but browser says yes
@@ -83,7 +77,7 @@ export default function Home() {
     updateStepData("Connect device", { status: "success" });
 
     appendStep({ name: "Download firmware", status: "running", progress: -1 });
-    const firmwareFile = await downloadEnglishFirmware().catch((e) => {
+    const firmwareFile = await getFirmware("3.0.8").catch((e) => {
       updateStepData("Download firmware", {
         status: "failed",
         errorMessage: e.toString(),
diff --git a/tsconfig.json b/tsconfig.json
index cf9c65d..117e4bb 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -13,6 +13,7 @@
     "isolatedModules": true,
     "jsx": "react-jsx",
     "incremental": true,
+    "noUncheckedIndexedAccess": true,
     "plugins": [
       {
         "name": "next"