Power BI REST API Export File In Group JSON deserializing error

Hello:

I am trying to use PowerBI REST API (in C#) using 

  using var response = await _httpClient.SendAsync(httpRequest, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
... 

if(response.StatusCode = HttpStatusCode.Accepted){
  var result =  await response.Content.ReadFromJsonAsync<Export>(cancellationToken: cancellationToken) 

...}

 I got response back, but when I deserialize json , it throws exception 

The JSON value could not be converted to System.Nullable`1[Microsoft.PowerBI.Api.Models.ExportState]. Path: $.status | LineNumber: 1 | BytePositionInLine: 278.

Can someone help with this error?

When I used online tryit from https://docs.microsoft.com/en-us/rest/api/power-bi/reports/exporttofileingroup#code-try-0 , I noticed the response is like this:

{
  "@odata.context": "http://.../v1.0/myorg/groups/.../$metadata#exports/$entity",
  "id": "...",
  "createdDateTime": "2021-05-05T...Z",
  "lastActionDateTime": "2021-05-05T...Z",
  "reportId": "...",
  "status": "NotStarted",
  "percentComplete": 0,
  "expirationTime": "0001-01-01T00:00:00Z"
}