How to pass S3 object and Key to HumanLoopInput of start_human_loop

I am able to extract text from my multi-page PDF using Amazon Textract. Now I want start Human Loop Review. I have already created a workflow and specified the condition there to trigger the Human Loop. Below is my code: -

import os
import json
import time
import uuid
from urllib.parse import unquote_plus
import boto3

def lambda_handler(event, context):
    textract = boto3.client("textract")
    a2i = boto3.client("sagemaker-a2i-runtime")
    FLOW_ARN = os.environ["FLOW_ARN"]
    if event:
        file_obj = event["Records"][0]
        bucketname = str(file_obj["s3"]["bucket"]["name"])
        filename = unquote_plus(str(file_obj["s3"]["object"]["key"]))
        
        # Start document analysis for the whole document
        response = textract.start_document_analysis(
            DocumentLocation={
                "S3Object": {
                    "Bucket": bucketname,
                    "Name": filename,
                }
            },
            FeatureTypes=["FORMS"],  # Specify the feature types to analyze
            ClientRequestToken=str(uuid.uuid4()),  # Generate a unique client request token
        )
        
        # Retrieve the job ID from the response
        job_id = response["JobId"]
        
        # Poll for the completion of the job
        while True:
            job_status = textract.get_document_analysis(JobId=job_id)['JobStatus']
            if job_status in ['SUCCEEDED', 'FAILED']:
                break
            time.sleep(5)  # Wait for 5 seconds before checking again
        
        # Get the results of the analysis
        response = textract.get_document_analysis(JobId=job_id)
        
        # Process the results
        print(json.dumps(response))
        
        a2i.start_human_loop(
            HumanLoopName=uuid.uuid4().hex,
            FlowDefinitionArn=FLOW_ARN,
            HumanLoopInput={
                'InputContent': json.dumps({
                    "InitialValue": {
                        "Bucket": bucketname,
                            "DocumentPath": filename,
                         }
                    })
                },
                DataAttributes={
                    'ContentClassifiers': [
                        'FreeOfAdultContent',
                    ]
                }
            )

        return {
            "statusCode": 200,
            "body": json.dumps("Document processed successfully!"),
        }

    return {"statusCode": 500, "body": json.dumps("Issue processing file!")}

I was expecting it to start the human loop review but it return following error: -

[ERROR] ValidationException: An error occurred (ValidationException) when calling the StartHumanLoop operation: Provided InputContent is not valid. Please use valid InputContent JSON and try your request again.

Could someone please point what I am doing wrong? I need to pass my PDF in S3 bucket to HumanLoopInput.

Thanks