Getting DisplayText attribute as null from Speech To Text Rest API while using from Postman

Question

This is the request I am sending:-
{"uri":"https://australiaeast.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1?language=en-US","method":"POST","headers":{"Content-Type":"audio/wav","Ocp-Apim-Subscription-Key":"ValidKey"},"body":"base64String"}
and getting below response:-
{
"RecognitionStatus": "Success",
"Offset": 300000,
"Duration": 301200000,
"DisplayText": ""
}

Could you please help me in understanding why displayText is null. PS the audio file is wav and has clear speech