diff --git a/README.md b/README.md
index b007e59..a7edc17 100644
--- a/README.md
+++ b/README.md
@@ -21,9 +21,7 @@ yarn add socket.io-middleware-firebase-admin
 API
 ---
 
-## createIoMiddlewareFirebaseAdmin({ credential }, (options = {})): middleware
-
-[initialize "Firebase Admin SDK" using `credential`](https://firebase.google.com/docs/admin/setup)
+## createIoMiddlewareFirebaseAdmin(app, (options = {})): middleware
 
 ```js
 import { createServer } from "http";
@@ -35,7 +33,10 @@ import credential from "./.credential";
 
 const server = createServer();
 const io = createIoServer(server);
-const ioMiddleware = createIoMiddlewareFirebaseAdmin({ credential });
+const app = firebaseAdmin.initializeApp({
+  credential: firebaseAdmin.credential.cert(credential)
+});
+const ioMiddleware = createIoMiddlewareFirebaseAdmin(app);
 
 io.use(ioMiddleware);
 
diff --git a/index.mjs b/index.mjs
index 72858fa..918d08a 100644
--- a/index.mjs
+++ b/index.mjs
@@ -1,11 +1,7 @@
-import firebaseAdmin from "firebase-admin";
 import cookie from "cookie";
 
-export default ({ credential }, options = {}) => {
-  const app = firebaseAdmin.initializeApp({
-    credential: firebaseAdmin.credential.cert(credential)
-  });
-  const auth = firebaseAdmin.auth();
+export default (app, options = {}) => {
+  const auth = app.auth();
   const opts = Object.assign({ deny: true, cache: true }, options);
 
   const cache = {};
@@ -41,9 +37,6 @@ export default ({ credential }, options = {}) => {
   middleware.deleteCache = key => {
     delete cache[key];
   };
-  middleware.firebaseAdminClose = () => {
-    app.delete();
-  };
 
   middleware.extra = {
     verifyCustomTokenUrl:
diff --git a/package.json b/package.json
index e893b04..16c4172 100644
--- a/package.json
+++ b/package.json
@@ -10,8 +10,7 @@
     "postversion": "git push --follow-tags && conventional-github-releaser -p atom"
   },
   "dependencies": {
-    "cookie": "^0.3.1",
-    "firebase-admin": "^6.0.0"
+    "cookie": "^0.3.1"
   },
   "devDependencies": {
     "@babel/cli": "^7.0.0-rc.1",
@@ -22,6 +21,7 @@
     "babel-plugin-add-module-exports": "^0.3.3",
     "bluebird": "^3.5.1",
     "eastern": "^0.1.0",
+    "firebase-admin": "^6.0.0",
     "prettier": "^1.14.2",
     "request": "^2.88.0",
     "server-destroy": "^1.0.1",
diff --git a/test.mjs b/test.mjs
index ab6d3f4..c7f6f29 100644
--- a/test.mjs
+++ b/test.mjs
@@ -8,17 +8,21 @@ import {
   createIoClientAsync,
   createSession
 } from "./test-helpers";
+import firebaseAdmin from "firebase-admin";
 import createFirebaseAdminMiddlware from "./";
 import credential from "./.credential";
 
-let middleware, server, port;
+let app, middleware, server, port;
 spec.before(async () => {
-  middleware = createFirebaseAdminMiddlware({ credential });
+  app = firebaseAdmin.initializeApp({
+    credential: firebaseAdmin.credential.cert(credential)
+  });
+  middleware = createFirebaseAdminMiddlware(app);
   server = await createIoServerAsync(middleware);
   port = server.address().port;
 });
 spec.after(async () => {
-  middleware.firebaseAdminClose();
+  app.delete();
   server.destroy();
 });