Evaluate DMN Decision via REST API

I wanted to evaluate a DMN decision via Postman but was getting following error:

Error Message
{

“type”: “RestException”,

“message”: “Cannot evaluate decision beverages:1:9168ea24-86fa-11eb-9966-0242ac100802: Exception while evaluating decision with key ‘null’”

}

Request Detail
POST: <>/engine-rest/decision-definition/beverages:1:9168ea24-86fa-11eb-9966-0242ac100802/evaluate

Request Body:
{

“variables” : {

“desiredDish” : { “value” : “Spareribs”, “type” : “String” },

“guestsWithChildren” : { “value” : true , “type” : “Boolean”}

}

}