How to provide instance identity document for registering the container instance to AWS ECS Cluster using AWS Javascript SDK?

I'm using AWS Javascript SDK to create a cluster and register the container instance for ECS task. I got struck by providing the instance identity document value for the request parameters. I'm trying to register my running EC2 instance as container instance. I got the instance identity document by executing the following command curl command.

curl http://169.254.169.254/latest/dynamic/instance-identity/document/

Output:

{
  "accountId" : "01234567891",
  "architecture" : "x86_64",
  "availabilityZone" : "us-east-11",
  "billingProducts" : null,
  "devpayProductCodes" : null,
  "marketplaceProductCodes" : null,
  "imageId" : "ami-123456789012d",
  "instanceId" : "i-12345678901",
  "instanceType" : "t2.micro",
  "kernelId" : null,
  "pendingTime" : "2020-07-12T15:57:19Z",
  "privateIp" : "172.31.33.40",
  "ramdiskId" : null,
  "region" : "us-east-1",
  "version" : "2017-09-30"
}

I have converted the output to string but I'm getting an error in NodeJS code.

{ InvalidParameterException: An identity document was provided, but not valid.

message:
'The identity document and identity document signature were not valid.',
code: 'InvalidParameterException',
time: 2020-07-14T07:30:59.782Z,
requestId: '7bca816f-8c8f-4e6e-8e77-04c93b45c17e',
statusCode: 400,
retryable: false,
retryDelay: 68.57527460306602 } 'InvalidParameterException: The identity document and identity document signature were not valid.\n  

Do we have to execute registerContainerInstances API from an EC2 instance?

Please guide!!!