Flutter API请求BadState响应

我目前正在try 使用Ffltter实现一个API请求.我已经在Postman中实现了Resuest，并try 了所有数据.结果是成功的.然后我试图在我的计划中实现这个要求.然而，我这里有一个问题，这就是为什么我总是得到ResponseCode400.然而，我再也无法解释为什么.我在用这里的FatSecret API元.

根据文档，我用于此请求的代码需要授权头.它由客户ID和客户秘密组成.正文中还包含"GRANT_TYPE":"CLIENT_CREDICATIONS"和"SCOPE":"PRIMILER".

class FatSecretAPI extends ConsumerWidget {
  const FatSecretAPI({super.key});

final String clientId = "...";
final String clientSecret = "...";

  Future<FatSecretToken> fetchAccesToken() async {
    final response = await http
        .post(Uri.parse("https://oauth.fatsecret.com/connect/token"),
        body: {
      "grant_type": "client_credentials",
      "scope": "premier"
    }, headers: {
           HttpHeaders.authorizationHeader: "Basic ${base64.encode(utf8.encode('$clientId:$clientSecret'))};"
    });
    if (response.statusCode == 200) {
      var object = FatSecretToken.fromJson(
          jsonDecode(response.body) as Map<String, dynamic>);
      print(object.accessToken);
      return object;
    } else {
      print(response.statusCode);
      throw Exception("Failed to load token");
    }
  }

  Future<void> loadToken(WidgetRef ref) async {
    final token = await fetchAccesToken();
    ref.read(accesTokenProvider.notifier).state = token.accessToken;
  }
}

class FatSecretToken {
  final String accessToken;
  final String tokenType;
  final int expiresIn;

  FatSecretToken(
      {required this.accessToken,
      required this.tokenType,
      required this.expiresIn});

  factory FatSecretToken.fromJson(Map<String, dynamic> json) {
    return FatSecretToken(
      accessToken: json['access_token'],
      tokenType: json['token_type'],
      expiresIn: json['expires_in'],
    );
  }
}

Json格式的响应实际上应该出现在这里，它包含有关accesToken和令牌类型的信息.