API call via AWS API Gateway return 403 after adding headers

Question

I have the requirement to invoke an API call via AWS API Gateway. The API request is signed using the Sigv4 algorithm and using Authorization Headers https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-auth-using-authorization-header.html

This has been working perfectly fine until I added to headers which are necessary for my API call. Since, then I get 403 from API Gateway.

The code is in python as below:

    def __init__(self, region='my-region'):
        self.region = region
        self.headers = {
            'tracking-id': '96fe5650-271e-4808-85d3-4fcb3ab34967',
            'session-id': '96fe5650-271e-4808-85d3-4fcb3ab34967'
        }

def sign_url(self, url, credentials, action='execute-api', method='GET'):
        """ Signs request with sig4. Doesn't support request bodies
            :returns AWS request, prepared with url and signed headers
        """
        auth_signer = SigV4Auth(credentials, action, self.region)
        request = AWSRequest(method=method, url=url, headers=self.headers())
        auth_signer.add_auth(request)
        return request.prepare()

def invoke_my_api(self):
   signed_prepared_request = self.iam_authorizer.sign_url(target_url, 
   self.api_credentials.get_frozen_credentials())

   self.client.get(my_uri, headers=signed_prepared_request.headers, catch_response=True) 
   as response:
            if response.status_code == 404:
                response.success()

Am I adding headers in incorrect method or what could be the problem?

Without headers here: AWSRequest(method=method, url=url) works perfectly fine. But adding headers is messing up something!