Lambda (inside VPC) using Neptune API results in error: EndpointConnectionError: Could not connect to the endpoint URL

I have a lambda and a neptune DB in the same VPC. I am trying to read from the neptune DB stream, but when running the lambda I am getting a connection error.

EndpointConnectionError: Could not connect to the endpoint URL: \"https://neptuneclusteridinstance123-asdf123.abcd.us-east-1.neptune.amazonaws.com:8182/propertygraph/stream?limit=100&iteratorType=AT_SEQUENCE_NUMBER&commitNum=0\

Any ideas on what is going wrong?

CDK:

const vpc = new ec2.Vpc(this, "vpc", {
      ipAddresses: ec2.IpAddresses.cidr('10.0.0.0/20'),
      natGateways: 1,
      maxAzs: 2,
      subnetConfiguration: [
        {
          name: 'private-subnet-1',
          subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS,
          cidrMask: 26
        },
        {
          name: 'public-subnet-1',
          subnetType: ec2.SubnetType.PUBLIC,
          cidrMask: 26
        }
      ]
    })

    // Get lists of Subnets by type
    var privateSubnets = vpc.privateSubnets;

    // Create Subnet group list to be used with Neptune.
    const neptuneSubnets: ec2.SubnetSelection = { subnets: privateSubnets };

    const subnetGroup = new neptune.SubnetGroup(this, 'subnet_group', {
      vpc: vpc,
      vpcSubnets: {
        subnets: privateSubnets
      }
    })

    // Neptune cluster parameter group
    const neptuneClusterParameterGroup = new neptune.ClusterParameterGroup(this, 'neptuneClusterParameterGroup', {
      description: "the main purpose of this parameter group is to enable neptune streams",
      family: neptune.ParameterGroupFamily.NEPTUNE_1_3,
      parameters: {
        neptune_streams: '1'
      }
    })

    const neptune_cluster = new neptune.DatabaseCluster(this, 'neptune_cluster_id', {
      vpc: vpc,
      vpcSubnets: neptuneSubnets,
      subnetGroup: subnetGroup,
      instanceType: neptune.InstanceType.T3_MEDIUM,
      deletionProtection: false,
      clusterParameterGroup: neptuneClusterParameterGroup
    })

    // lambda
    const neptuneLambda = new lambda.Function(this, "neptuneLambda", {
      vpc: vpc,
      vpcSubnets: {
        subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS
      },
      memorySize: 512, // MB
      runtime: lambda.Runtime.PYTHON_3_8,
      code: lambda.Code.fromAsset(path.join(__dirname, '../src/lambda')),
      handler: 'neptuneLambdaHandler.handler',
      timeout: cdk.Duration.seconds(120) 
    })

Lambda:

neptuneData = boto3.client(
    'neptunedata',
    endpoint_url=neptune_endpoint_URL)

response = neptuneData.get_propertygraph_stream(
        limit=100,
        iteratorType='AT_SEQUENCE_NUMBER',
        commitNum=int(startingCommitNum)
    )