What is Boto3 Inspector v2 list_findings' nextToken initial value?

I am using boto3 Python to list findings from Inspector v2, using the list_findings() method inside a loop, according to AWS Boto3 Inspector2 Docs I have to set the value of this parameter to null for the first request to a list action but keep getting error in all these cases for variable next_token:

• nextToken = None: botocore.exceptions.ParamValidationError: Parameter validation failed: Invalid type for parameter nextToken, value: None, type: <class 'NoneType'>, valid types: <class 'str'>

• nextToken = 'null': botocore.errorfactory.ValidationException: An error occurred (ValidationException) when calling the ListFindings operation: Pagination token exception in operation 'ListFindings': invalid payload wrapping schema -24855

• nextToken = empty string: An error occurred (ValidationException) when calling the ListFindings operation: Pagination token exception in operation 'ListFindings': invalid payload (no encryption schema version)

here is my code:

import boto3
inspector = boto3.client("inspector2")
next_token = "" # I change the value of this variable 
while True:
    response = inspector.list_findings(
            filterCriteria={
                'findingStatus': [
                    {
                        'comparison': 'EQUALS',
                        'value': 'ACTIVE'
                    },
                ],
                'findingType': [
                    {
                        'comparison': 'EQUALS',
                        'value': 'PACKAGE_VULNERABILITY'
                    },
                ],
            },
        nextToken=next_token
    )
    next_token= response.get("nextToken") 
    
    # Some Code Here 
    
    if next_token == None:
        break

I am confused about what should the value of nextToken be for the first request?

It would be easier to use paginate as explained in AWS docs:

# Create a client
inspector = boto3.client("inspector2")

# Create a reusable Paginator
paginator = inspector.get_paginator('list_findings')

# Create a PageIterator from the Paginator
page_iterator = paginator.paginate(filterCriteria={
                'findingStatus': [
                    {
                        'comparison': 'EQUALS',
                        'value': 'ACTIVE'
                    },
                ],
                'findingType': [
                    {
                        'comparison': 'EQUALS',
                        'value': 'PACKAGE_VULNERABILITY'
                    },
                ],
            })

for page in page_iterator:
    print(page)