diff --git a/jwt.go b/jwt.go
index 78d9553..28161ca 100644
--- a/jwt.go
+++ b/jwt.go
@@ -135,29 +135,28 @@
 		return nil, fmt.Errorf("oauth2: cannot fetch token: %v\nResponse: %s", resp.Status, body)
 	}
 
-	b := &tokenRespBody{}
-	if err := json.Unmarshal(body, b); err != nil {
+	b := make(map[string]interface{})
+	if err := json.Unmarshal(body, &b); err != nil {
 		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
 	}
-
-	token := &Token{
-		AccessToken: b.AccessToken,
-		TokenType:   b.TokenType,
-		Subject:     existing.Subject,
+	token := &Token{}
+	token.AccessToken, _ = b["access_token"].(string)
+	token.TokenType, _ = b["token_type"].(string)
+	token.Subject = existing.Subject
+	token.raw = b
+	if e, ok := b["expires_in"].(int); ok {
+		token.Expiry = time.Now().Add(time.Duration(e) * time.Second)
 	}
-
-	if b.IdToken != "" {
+	if idtoken, ok := b["id_token"].(string); ok {
 		// decode returned id token to get expiry
 		claimSet := &jws.ClaimSet{}
-		claimSet, err = jws.Decode(b.IdToken)
+		claimSet, err = jws.Decode(idtoken)
 		if err != nil {
 			return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
 		}
 		token.Expiry = time.Unix(claimSet.Exp, 0)
 		return token, nil
 	}
-
-	token.Expiry = time.Now().Add(time.Duration(b.ExpiresIn) * time.Second)
 	return token, nil
 }
 
diff --git a/jwt_test.go b/jwt_test.go
new file mode 100644
index 0000000..7d4711f
--- /dev/null
+++ b/jwt_test.go
@@ -0,0 +1,113 @@
+// Copyright 2014 The oauth2 Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package oauth2
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+var dummyPrivateKey = []byte(`-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----`)
+
+func TestJWTFetch_JSONResponse(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"access_token": "90d64460d14870c08c81352a05dedd3465940a7c", "scope": "user", "token_type": "bearer"}`))
+	}))
+	defer ts.Close()
+	conf, err := NewJWTConfig(&JWTOptions{
+		Email:      "aaa@xxx.com",
+		PrivateKey: dummyPrivateKey,
+	}, ts.URL)
+	tok, err := conf.FetchToken(nil)
+	if err != nil {
+		t.Errorf("Failed retrieving token: %s.", err)
+	}
+	if tok.Expired() {
+		t.Errorf("Token shouldn't be expired.")
+	}
+	if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
+	}
+	if tok.TokenType != "bearer" {
+		t.Errorf("Unexpected token type, %#v.", tok.TokenType)
+	}
+	scope := tok.Extra("scope")
+	if scope != "user" {
+		t.Errorf("Unexpected value for scope: %v", scope)
+	}
+}
+
+func TestJWTFetch_BadResponse(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"scope": "user", "token_type": "bearer"}`))
+	}))
+	defer ts.Close()
+	conf, err := NewJWTConfig(&JWTOptions{
+		Email:      "aaa@xxx.com",
+		PrivateKey: dummyPrivateKey,
+	}, ts.URL)
+	tok, err := conf.FetchToken(nil)
+	if err != nil {
+		t.Errorf("Failed retrieving token: %s.", err)
+	}
+	if tok.AccessToken != "" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
+	}
+	if tok.TokenType != "bearer" {
+		t.Errorf("Unexpected token type, %#v.", tok.TokenType)
+	}
+	scope := tok.Extra("scope")
+	if scope != "user" {
+		t.Errorf("Unexpected value for scope: %v", scope)
+	}
+}
+
+func TestJWTFetch_BadResponseType(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"access_token":123, "scope": "user", "token_type": "bearer"}`))
+	}))
+	defer ts.Close()
+	conf, err := NewJWTConfig(&JWTOptions{
+		Email:      "aaa@xxx.com",
+		PrivateKey: dummyPrivateKey,
+	}, ts.URL)
+	tok, err := conf.FetchToken(nil)
+	if err != nil {
+		t.Errorf("Failed retrieving token: %s.", err)
+	}
+	if tok.AccessToken != "" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
+	}
+}
diff --git a/oauth2.go b/oauth2.go
index 7f0571e..ac3072f 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -20,15 +20,6 @@
 	"time"
 )
 
-type tokenRespBody struct {
-	AccessToken  string `json:"access_token"`
-	TokenType    string `json:"token_type"`
-	RefreshToken string `json:"refresh_token"`
-	ExpiresIn    int64  `json:"expires_in"` // in seconds
-	Expires      int64  `json:"expires"`    // in seconds. Facebook returns expires_in as expires.
-	IdToken      string `json:"id_token"`
-}
-
 // TokenFetcher refreshes or fetches a new access token from the
 // provider. It should return an error if it's not capable of
 // retrieving a token.
@@ -212,7 +203,8 @@
 		return nil, fmt.Errorf("oauth2: cannot fetch token: %v\nResponse: %s", r.Status, body)
 	}
 
-	resp := &tokenRespBody{}
+	token := &Token{}
+	expires := int(0)
 	content, _, _ := mime.ParseMediaType(r.Header.Get("Content-Type"))
 	switch content {
 	case "application/x-www-form-urlencoded", "text/plain":
@@ -220,44 +212,45 @@
 		if err != nil {
 			return nil, err
 		}
-		resp.AccessToken = vals.Get("access_token")
-		resp.TokenType = vals.Get("token_type")
-		resp.RefreshToken = vals.Get("refresh_token")
-		resp.ExpiresIn, _ = strconv.ParseInt(vals.Get("expires_in"), 10, 64)
-		resp.Expires, _ = strconv.ParseInt(vals.Get("expires"), 10, 64)
-		resp.IdToken = vals.Get("id_token")
+		token.AccessToken = vals.Get("access_token")
+		token.TokenType = vals.Get("token_type")
+		token.RefreshToken = vals.Get("refresh_token")
+		token.raw = vals
+		e := vals.Get("expires_in")
+		if e == "" {
+			// TODO(jbd): Facebook's OAuth2 implementation is broken and
+			// returns expires_in field in expires. Remove the fallback to expires,
+			// when Facebook fixes their implementation.
+			e = vals.Get("expires")
+		}
+		expires, _ = strconv.Atoi(e)
 	default:
-		if err = json.Unmarshal(body, &resp); err != nil {
+		b := make(map[string]interface{})
+		if err = json.Unmarshal(body, &b); err != nil {
 			return nil, err
 		}
-	}
-	token := &Token{
-		AccessToken:  resp.AccessToken,
-		TokenType:    resp.TokenType,
-		RefreshToken: resp.RefreshToken,
+		token.AccessToken, _ = b["access_token"].(string)
+		token.TokenType, _ = b["token_type"].(string)
+		token.RefreshToken, _ = b["refresh_token"].(string)
+		token.raw = b
+		e, ok := b["expires_in"].(int)
+		if !ok {
+			// TODO(jbd): Facebook's OAuth2 implementation is broken and
+			// returns expires_in field in expires. Remove the fallback to expires,
+			// when Facebook fixes their implementation.
+			e, _ = b["expires"].(int)
+		}
+		expires = e
 	}
 	// Don't overwrite `RefreshToken` with an empty value
 	// if this was a token refreshing request.
-	if resp.RefreshToken == "" {
+	if token.RefreshToken == "" {
 		token.RefreshToken = v.Get("refresh_token")
 	}
-	if resp.ExpiresIn == 0 || resp.Expires == 0 {
+	if expires == 0 {
 		token.Expiry = time.Time{}
-	}
-	if resp.ExpiresIn > 0 {
-		token.Expiry = time.Now().Add(time.Duration(resp.ExpiresIn) * time.Second)
-	}
-	if resp.Expires > 0 {
-		// TODO(jbd): Facebook's OAuth2 implementation is broken and
-		// returns expires_in field in expires. Remove the fallback to expires,
-		// when Facebook fixes their implementation.
-		token.Expiry = time.Now().Add(time.Duration(resp.Expires) * time.Second)
-	}
-	if resp.IdToken != "" {
-		if token.Extra == nil {
-			token.Extra = make(map[string]string)
-		}
-		token.Extra["id_token"] = resp.IdToken
+	} else {
+		token.Expiry = time.Now().Add(time.Duration(expires) * time.Second)
 	}
 	return token, nil
 }
diff --git a/oauth2_test.go b/oauth2_test.go
index c231ff9..d633f86 100644
--- a/oauth2_test.go
+++ b/oauth2_test.go
@@ -84,14 +84,18 @@
 		t.Errorf("Token shouldn't be expired.")
 	}
 	if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
-		t.Errorf("Wrong access token, %#v.", tok.AccessToken)
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
 	}
 	if tok.TokenType != "bearer" {
-		t.Errorf("Wrong token type, %#v.", tok.TokenType)
+		t.Errorf("Unexpected token type, %#v.", tok.TokenType)
+	}
+	scope := tok.Extra("scope")
+	if scope != "user" {
+		t.Errorf("Unexpected value for scope: %v", scope)
 	}
 }
 
-func TestExchangeRequest_JsonResponse(t *testing.T) {
+func TestExchangeRequest_JSONResponse(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.URL.String() != "/token" {
 			t.Errorf("Unexpected exchange request URL, %v is found.", r.URL)
@@ -124,10 +128,46 @@
 		t.Errorf("Token shouldn't be expired.")
 	}
 	if tok.AccessToken != "90d64460d14870c08c81352a05dedd3465940a7c" {
-		t.Errorf("Wrong access token, %#v.", tok.AccessToken)
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
 	}
 	if tok.TokenType != "bearer" {
-		t.Errorf("Wrong token type, %#v.", tok.TokenType)
+		t.Errorf("Unexpected token type, %#v.", tok.TokenType)
+	}
+	scope := tok.Extra("scope")
+	if scope != "user" {
+		t.Errorf("Unexpected value for scope: %v", scope)
+	}
+}
+
+func TestExchangeRequest_BadResponse(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"scope": "user", "token_type": "bearer"}`))
+	}))
+	defer ts.Close()
+	conf := newTestConf(ts.URL)
+	tok, err := conf.Exchange("exchange-code")
+	if err != nil {
+		t.Errorf("Failed retrieving token: %s.", err)
+	}
+	if tok.AccessToken != "" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
+	}
+}
+
+func TestExchangeRequest_BadResponseType(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"access_token":123,  "scope": "user", "token_type": "bearer"}`))
+	}))
+	defer ts.Close()
+	conf := newTestConf(ts.URL)
+	tok, err := conf.Exchange("exchange-code")
+	if err != nil {
+		t.Errorf("Failed retrieving token: %s.", err)
+	}
+	if tok.AccessToken != "" {
+		t.Errorf("Unexpected access token, %#v.", tok.AccessToken)
 	}
 }
 
diff --git a/transport.go b/transport.go
index 706947b..4483c9c 100644
--- a/transport.go
+++ b/transport.go
@@ -6,6 +6,7 @@
 
 import (
 	"net/http"
+	"net/url"
 	"sync"
 	"time"
 )
@@ -30,14 +31,28 @@
 	// The remaining lifetime of the access token.
 	Expiry time.Time `json:"expiry,omitempty"`
 
-	// Extra optionally contains extra metadata from the server
-	// when updating a token. The only current key that may be
-	// populated is "id_token". It may be nil and will be
-	// initialized as needed.
-	Extra map[string]string `json:"extra,omitempty"`
-
 	// Subject is the user to impersonate.
 	Subject string `json:"subject,omitempty"`
+
+	// raw optionally contains extra metadata from the server
+	// when updating a token.
+	raw interface{}
+}
+
+// Extra returns an extra field returned from the server during token retrieval.
+// E.g.
+//     idToken := token.Extra("id_token")
+//
+func (t *Token) Extra(key string) string {
+	if vals, ok := t.raw.(url.Values); ok {
+		return vals.Get(key)
+	}
+	if raw, ok := t.raw.(map[string]interface{}); ok {
+		if val, ok := raw[key].(string); ok {
+			return val
+		}
+	}
+	return ""
 }
 
 // Expired returns true if there is no access token or the
@@ -105,17 +120,7 @@
 func (t *Transport) Token() *Token {
 	t.mu.RLock()
 	defer t.mu.RUnlock()
-	if t.token == nil {
-		return nil
-	}
-	return &Token{
-		AccessToken:  t.token.AccessToken,
-		TokenType:    t.token.TokenType,
-		RefreshToken: t.token.RefreshToken,
-		Expiry:       t.token.Expiry,
-		Extra:        t.token.Extra,
-		Subject:      t.token.Subject,
-	}
+	return t.token
 }
 
 // SetToken sets a token to the transport in a thread-safe way.