when STS is active authenticating to Polly using access key id credentials fails. Why?

[davehorton]

I am authenticating to Polly like so:

// credentials: {accessKeyId, secretAccessKey, region}
const polly = new Polly(credentials);
polly.synthesizeSpeech(opts, (err, data) => {..

This has worked fine on lots of installs. Now I am running on an EC2 instance for a customer of mine that has activated STS. I find that this call above now fails with this stack trace:

"stack":"ConfigError: Missing region in config\n    
at Request.VALIDATE_REGION (/opt/app/node_modules/aws-sdk/lib/event_listeners.js:94:47)\n    
at Request.callListeners (/opt/app/node_modules/aws-sdk/lib/sequential_executor.js:106:20)\n    
at callNextListener (/opt/app/node_modules/aws-sdk/lib/sequential_executor.js:96:12)\n    
at /opt/app/node_modules/aws-sdk/lib/event_listeners.js:86:9\n    
at finish (/opt/app/node_modules/aws-sdk/lib/config.js:396:7)\n    
at /opt/app/node_modules/aws-sdk/lib/config.js:414:9\n    
at Credentials.get (/opt/app/node_modules/aws-sdk/lib/credentials.js:127:7)\n    
at getAsyncCredentials (/opt/app/node_modules/aws-sdk/lib/config.js:408:24)\n    
at Config.getCredentials (/opt/app/node_modules/aws-sdk/lib/config.js:428:9)\n    
at Request.VALIDATE_CREDENTIALS (/opt/app/node_modules/aws-sdk/lib/event_listeners.js:81:26)","code":"ConfigError","time":"2022-08-04T08:26:15.493Z"},"msg":"Error synthesizing tts"}

I can see that this error is burped from with the STS code in the library, here

I'm not quite clear on why this is not working. Basically, I have a valid (access key id, secret access key, region} tuple that was generated by another AWS account, and want to use them on this EC2 instance for Polly TTS, but it seems like somehow because STS is activated I can't. Does anyone have an idea why this is, or how to resolve it?