diff --git a/index.js b/index.js
index b095060..62aabce 100644
--- a/index.js
+++ b/index.js
@@ -2,7 +2,6 @@
 
 const debug = require('debug')('koa-session');
 const ContextSession = require('./lib/context');
-const util = require('./lib/util');
 const assert = require('assert');
 const uid = require('uid-safe');
 const is = require('is-type-of');
@@ -70,14 +69,6 @@ function formatOpts(opts) {
 
   debug('session options %j', opts);
 
-  // setup encoding/decoding
-  if (typeof opts.encode !== 'function') {
-    opts.encode = util.encode;
-  }
-  if (typeof opts.decode !== 'function') {
-    opts.decode = util.decode;
-  }
-
   const store = opts.store;
   if (store) {
     assert(is.function(store.get), 'store.get must be function');
diff --git a/lib/context.js b/lib/context.js
index f1dc702..8aed0ae 100644
--- a/lib/context.js
+++ b/lib/context.js
@@ -69,10 +69,20 @@ class ContextSession {
     const ctx = this.ctx;
     const opts = this.opts;
 
-    const externalKey = ctx.cookies.get(opts.key, opts);
-    debug('get external key from cookie %s', externalKey);
+    const cookie = ctx.cookies.get(opts.key, opts);
+    debug('get external key from cookie %s', cookie);
 
-    if (!externalKey) {
+    if (!cookie) {
+      // create a new `externalKey`
+      this.create();
+      return;
+    }
+    let externalKey = cookie;
+    try {
+      if (typeof opts.decode === 'function') {
+        externalKey = opts.decode(externalKey);
+      }
+    } catch (err) {
       // create a new `externalKey`
       this.create();
       return;
@@ -109,7 +119,7 @@ class ContextSession {
     let json;
     debug('parse %s', cookie);
     try {
-      json = opts.decode(cookie);
+      json = typeof opts.decode === 'function' ? opts.decode(cookie) : util.decode(cookie);
     } catch (err) {
       // backwards compatibility:
       // create a new session if parsing fails.
@@ -263,13 +273,17 @@ class ContextSession {
         changed,
         rolling: opts.rolling,
       });
-      this.ctx.cookies.set(key, externalKey, opts);
+      let cookie = externalKey;
+      if (typeof opts.encode === 'function') {
+        cookie = opts.encode(externalKey);
+      }
+      this.ctx.cookies.set(key, cookie, opts);
       return;
     }
 
     // save to cookie
     debug('save %j to cookie', json);
-    json = opts.encode(json);
+    json = typeof opts.encode === 'function' ? opts.encode(json) : util.encode(json);
     debug('save %s', json);
 
     this.ctx.cookies.set(key, json, opts);
diff --git a/test/cookie.test.js b/test/cookie.test.js
index dc15fb9..ddc2462 100644
--- a/test/cookie.test.js
+++ b/test/cookie.test.js
@@ -761,10 +761,10 @@ describe('Koa Session Cookie', () => {
     before(() => {
       app = App({ rolling: true });
 
-      app.use(function* () {
-        console.log(this.path);
-        if (this.path === '/set') this.session = { foo: 'bar' };
-        this.body = this.session;
+      app.use(async function(ctx) {
+        console.log(ctx.path);
+        if (ctx.path === '/set') ctx.session = { foo: 'bar' };
+        ctx.body = ctx.session;
       });
     });