error message":"`api_key` should be minimum 6 characters in length."

POST https://api.kite.trade/session/token

POST data:
{
"api_key": "XXXXXXXXXXXXXXX",
"request_token": "XXXXXXXXXXXXXXXXXXXXX",
"checksum": "XXXXXXXXXXXXXXXXX"
}

Headers:
Connection: keep-alive
X-Kite-Version: 3
Content-Type: application/json
Content-Length: 150
Host: api.kite.trade
User-Agent: Apache-HttpClient/4.5.10 (Java/11.0.6)

Response:
{"status":"error","message":"`api_key` should be minimum 6 characters in length.","data":null,"error_type":"InputException"}

I am specifying valid api_key which is of 16 characters along with valid request_token and checksum sha256 (api_key + request_token + api_secret)