Having issue while sending lambda response to Lex v2. Error : "Invalid Lambda Response: Can not construct instance of IntentResponse"

I am trying to create a bot in lex v2 console using c# .net function. All are done, in the integration part we have the following error in lex v2 chat window.

we are using custom models for Request and Response. Is there any AWS DLL for this? like LexEvent & LexResponse in lex v1

Invalid Lambda Response: Received invalid response from Lambda: Can not construct instance of IntentResponse, problem: The validated object is null at [Source: {"Jsonvalue"}; line: 1, column: 561]

Response we trying to return is,

{ "SessionId":"125512899102434", "SessionState": { "ActiveContexts":[], "SessionAttributes":{}, "DialogAction": { "SlotToElicit":"Patientid", "Type":"ElicitSlot" }, "Intent": { "ConfirmationState":"None", "Name":"ScheduleAppointment", "Slots": { "MobileNumber":null, "Department":null, "FirstName":null, "Patientid":null, "Time":null, "LastName":null, "Gender":null, "Doctor":null, "UserType":null, "Date":null, "Age":null, "Location":null }, "State":"InProgress" } }, "Messages": [ { "ContentType":"PlainText", "Content":"Please provide your Patient ID" } ] }

Please help us to resolve this issue and move forward. thanks, Mohan