diff --git a/functions/src/functions/create-custom-token.ts b/functions/src/functions/create-custom-token.ts
index 0a3a257..0740e1a 100644
--- a/functions/src/functions/create-custom-token.ts
+++ b/functions/src/functions/create-custom-token.ts
@@ -13,77 +13,84 @@ export function buildCreateCustomToken(
   app: App,
   region: string
 ): functions.HttpsFunction {
-  return functions.region(region).https.onCall(async (data, _context) => {
-    if (typeof data !== "object" || data === null) {
-      throw new functions.https.HttpsError(
-        "invalid-argument",
-        "The function must be called with one argument"
-      );
-    }
-    const deviceId = data.device_id;
-    const deviceSecret = data.device_secret;
-    if (typeof deviceId !== "string" || typeof deviceSecret !== "string") {
-      throw new functions.https.HttpsError(
-        "invalid-argument",
-        "The function must be called with one argument"
-      );
-    }
+  return functions
+    .region(region)
+    .https.onCall(async (data: unknown, _context) => {
+      if (
+        typeof data !== "object" ||
+        data === null ||
+        !("device_id" in data) ||
+        !("device_secret" in data)
+      ) {
+        throw new functions.https.HttpsError(
+          "invalid-argument",
+          "The function must be called with one argument"
+        );
+      }
+      const deviceId = data.device_id;
+      const deviceSecret = data.device_secret;
+      if (typeof deviceId !== "string" || typeof deviceSecret !== "string") {
+        throw new functions.https.HttpsError(
+          "invalid-argument",
+          "The function must be called with one argument"
+        );
+      }
 
-    type Device = {
-      encryptedSecret: string;
-      id: string;
-      uid: string;
-    };
-    const db = getFirestore(app);
-    const deviceSnapshot = await db
-      .collection("devices")
-      .doc(deviceId)
-      .withConverter({
-        fromFirestore: (snapshot: QueryDocumentSnapshot): Device => {
-          return snapshot.data() as Device;
-        },
-        toFirestore: (device: Device): DocumentData => {
-          return device;
-        },
-      })
-      .get();
-    const device = deviceSnapshot.data();
-    const uid =
-      device !== undefined &&
-      device.id === deviceId &&
-      (await compare(deviceSecret, device.encryptedSecret))
-        ? device.uid
-        : uuidv4();
-    const customToken = await getAuth(app).createCustomToken(uid);
-    await deviceSnapshot.ref.set({
-      id: deviceId,
-      encryptedSecret: await hash(deviceSecret, 10),
-      uid,
-    });
+      type Device = {
+        encryptedSecret: string;
+        id: string;
+        uid: string;
+      };
+      const db = getFirestore(app);
+      const deviceSnapshot = await db
+        .collection("devices")
+        .doc(deviceId)
+        .withConverter({
+          fromFirestore: (snapshot: QueryDocumentSnapshot): Device => {
+            return snapshot.data() as Device;
+          },
+          toFirestore: (device: Device): DocumentData => {
+            return device;
+          },
+        })
+        .get();
+      const device = deviceSnapshot.data();
+      const uid =
+        device !== undefined &&
+        device.id === deviceId &&
+        (await compare(deviceSecret, device.encryptedSecret))
+          ? device.uid
+          : uuidv4();
+      const customToken = await getAuth(app).createCustomToken(uid);
+      await deviceSnapshot.ref.set({
+        id: deviceId,
+        encryptedSecret: await hash(deviceSecret, 10),
+        uid,
+      });
 
-    // create user document
-    type UserDocument = {
-      id: string;
-      account_ids: string[];
-    };
-    const userSnapshot = await db
-      .collection("users")
-      .doc(uid)
-      .withConverter({
-        fromFirestore: (snapshot: QueryDocumentSnapshot): UserDocument => {
-          return snapshot.data() as UserDocument;
-        },
-        toFirestore: (user: UserDocument): DocumentData => {
-          return user;
-        },
-      })
-      .get();
-    if (!userSnapshot.exists) {
-      await userSnapshot.ref.create({ account_ids: [], id: uid });
-    }
+      // create user document
+      type UserDocument = {
+        id: string;
+        account_ids: string[];
+      };
+      const userSnapshot = await db
+        .collection("users")
+        .doc(uid)
+        .withConverter({
+          fromFirestore: (snapshot: QueryDocumentSnapshot): UserDocument => {
+            return snapshot.data() as UserDocument;
+          },
+          toFirestore: (user: UserDocument): DocumentData => {
+            return user;
+          },
+        })
+        .get();
+      if (!userSnapshot.exists) {
+        await userSnapshot.ref.create({ account_ids: [], id: uid });
+      }
 
-    return {
-      custom_token: customToken,
-    };
-  });
+      return {
+        custom_token: customToken,
+      };
+    });
 }