IAM issue following DynamoDB ‘local’ tutorial

[markedup]

I'm working through your Using AWS DynamoDB with Node.js, and working with the master-w-dynamodb branch as instructed.

I'm successfully running DynamoDB locally, and have gotten as far as being ready to POST my first record. However, whenever I try to send a JSON object via Postman, I get the following response:

{
    "__type": "com.amazonaws.dynamodb.v20120810#MissingAuthenticationToken",
    "message": "Request must contain either a valid (registered) AWS access key ID or X.509 certificate."
}

I don't really understand why this is happening, as I've followed the foo ... bar example in the article for setting up local creds.