使用API生成总账凭证时提示：对象必须实现 IConvertible

如题，我的代码如下：
g = "GL_VOUCHER";

f = "{\"Creator\":\"administrator\",\"NeedUpDateFields\":[],\"Model\":{" +
"\"FVOUCHERID\":\"0\"," +
"\"FAccountBookID\":{\"FNUMBER\" : \"001\"}," +
"\"FDATE\":\"2015-8-18\"," +
"\"FVOUCHERGROUPID\":{\"FNUMBER\":\"PRE001\"}," +
"\"FVOUCHERGROUPNO\":\"\"," +
"\"FEntity\":[{" +
"\"FEXPLANATION\":{\"FNUMBER\":\"XXXXXX\"}," +
"\"FACCOUNTID\":{\"FNUMBER\":\"2202.01\"}," +
//"\"FDetailID\":{\"FNUMBER\":\"\"}, "
"\"FCURRENCYID\":{\"FNUMBER\":\"PRE001\"}," +
"\"FEXCHANGERATETYPE\":{\"FNUMBER\":\"HLTX01_SYS\"}," +
"\"FEXCHANGERATE\":1," +
"\"FDEBIT\":100," +
"},{" +
"\"FEXPLANATION\":{\"FNUMBER\":\"asd\"}," +
"\"FACCOUNTID\":{\"FNUMBER\":\"1001\"}," +
//"\"FDetailID\":{\"FNUMBER\":\"\"}, "
"\"FCURRENCYID\":{\"FNUMBER\":\"PRE001\"}," +
"\"FEXCHANGERATETYPE\":{\"FNUMBER\":\"HLTX01_SYS\"}," +
"\"FEXCHANGERATE\":1," +
"\"FCREDIT\":100," +
"}]" +
"}}";

responseOut = client.Execute("Kingdee.BOS.WebApi.ServicesStub.DynamicFormService.Save", new object[] { g, f });

然后就报错：Kingdee.BOS.WebApi.Client.ServiceException (0x80004005): 对象必须实现 IConvertible。
求大神解答下是什么原因造成的。谢谢！