[SuiteScript 2.0 Restlet] Some exceptions are not json decodable

Hi NetSuite, we are using SuiteScript 2.0 restlet to do some crud operations with NS, and  we observe restlet exceptions are returned in json type, however we observe some exceptions are not valid json. e.g.

{"error" : {"code" : "DATE_EXPECTED", "message" : "{\\"type\\":\\"error.SuiteScriptError\\",\\"name\\":\\"DATE_EXPECTED\\",\\"message\\":\\"You entered \\\'null\\\' into a field where a calendar date was expected.\\\\nPlease go back and change this value to the correct date.\\",\\"id\\":\\"\\",\\"stack\\":[\\"Error\\n    at RecordInvoker.save (suitescript/resources/javascript/record/serverRecordService.js:371:13)\\n    at NetSuiteObject.thenableFunction() (suitescript/resources/javascript/record/proxy.js:115:24)\\n    at createRecord (/SuiteApps/com.ziphq.zipintegration/zip_integration.js:246:22)\\n    at Object.post (/SuiteApps/com.ziphq.zipintegration/zip_integration.js:333:16)\\"],\\"cause\\":{\\"type\\":\\"internal error\\",\\"code\\":\\"DATE_EXPECTED\\",\\"details\\":\\"You entered \\\'null\\\' into a field where a calendar date was expected.\\\\nPlease go back and change this value to the correct date.\\",\\"userEvent\\":null,\\"stackTrace\\":[\\"Error\\n    at RecordInvoker.save (suitescript/resources/javascript/record/serverRecordService.js:371:13)\\n    at NetSuiteObject.thenableFunction() (suitescript/resources/javascript/record/proxy.js:115:24)\\n    at createRecord (/SuiteApps/com.ziphq.zipintegration/zip_integration.js:246:22)\\n    at Object.post (/SuiteApps/com.ziphq.zipintegration/zip_integration.js:333:16)\\"],\\"notifyOff\\":false},\\"notifyOff\\":false,\\"userFacing\\":true}"}}