How to write item, with Null attribute, to Dynamodb through BatchWriteItem task in AWS Step function

I am attempting to use aws step functions with the batchwriteitem task for dynamodb.

Unfortunately, the step function throws an error suggesting "The field "NULL" is not supported by Step Functions". But this is how AWS SDK documentation suggests we label null values in API call. If the "NULL" value isn't expected, then how am I supposed to write an item, that has a Null attribute, to Dynamodb through BatchWriteItem task in AWS Step functions

Example task in Step function which is causing step function ASL error. I tried changing "NULL" to "Null" or "null". And none of these work?

"BatchWriteItem": {
    "Type": "Task",
    "End": true,
    "Parameters": {
        "RequestItems": {
            "sample-dynamodb-table": [
                {
                    "PutRequest": {
                        "Item": {
                            "PK": {
                                "S": "trash"
                            },
                            "SK": {
                                "S": "trash"
                            },
                            "value": {
                                "NULL": true
                            }
                        }
                    }
                }
            ]
        }
    },
    "Resource": "arn:aws:states:::aws-sdk:dynamodb:batchWriteItem"
}