See the integ test in
aws-s3-deployment/test/integ.bucket-deployment-substitution.ts for an example.

The assertions handler will attempt to log the response of the made API request. In this case it is S3.getObject which includes a Blob in the Body field.

When logging the response, the handler uses JSON.stringify() which fails with to the following error:

Converting circular structure to JSON
    --> starting at object with constructor 'TLSSocket'
    |     property 'parser' -> object with constructor 'HTTPParser'
    --- property 'socket' closes the circle (RequestId: 50c1b6cd-47d2-494f-baf1-d22646cd4e5f)

This might also conceal an issue with returning blob data from the ApiCall, but not sure yet.