ios – ACAccountCredential为oauthToken返回null

[accStore requestAccessToAccountsWithType:fBACcountType
                                           options:options
                                        completion:^(BOOL granted,NSError *error) {
                                            if (granted) {
                                                // If access granted,then get the Facebook account info
                                                NSArray *accounts = [accStore
                                                                     accountsWithAccountType:fBACcountType];
                                                ACAccount *acc = [accounts lastObject];

                                                // Get the access token,Could be used in other scenarios
                                                ACAccountCredential *fbCredential = [acc credential];
                                                NSString *accessToken = [fbCredential oauthToken];
                                                NSLog(@"Facebook Access Token: %@",accessToken);


                                                // Add code here to make an API request using the SLrequest class

                                            } else {
                                                NSLog(@"Access not granted");
                                            }
                                        }];

打印出：

Facebook Access Token: (null)

我被授予访问权限,但令牌为空.我究竟做错了什么？

谢谢