Lambda PutObjectCommand failing with "Resolved credential object is not valid"

I have a lambda which is attempting to put an object in an S3 bucket.

The code to configure the s3 client is as follows:

const configuration: S3ClientConfig = {
  region: 'us-west-2',
};

if (process.env.DEVELOPMENT_MODE) {
  configuration.credentials = {
    accessKeyId: process.env.AWS_ACCESS_KEY!,
    secretAccessKey: process.env.AWS_SECRET_KEY!,
  }
}

export const s3 = new S3Client(configuration);

And the code to upload the file is as follows:

s3.send(new PutObjectCommand({
  Bucket: bucketName,
  Key: fileName,
  ContentType: contentType,
  Body: body,
}))

This works locally. The lambda's role includes a policy which in turn includes the following statement:

{
    "Action": [
        "s3:DeleteObject",
        "s3:PutObject"
    ],
    "Resource": [
        "arn:aws:s3:::BUCKET_NAME/*"
    ],
    "Effect": "Allow"
}

However, when I invoke this lambda, it fails with the following stack trace

Error: Resolved credential object is not valid
    at SignatureV4.validateResolvedCredentials (webpack://backend/../node_modules/@aws-sdk/signature-v4-multi-region/node_modules/@aws-sdk/signature-v4/dist-es/SignatureV4.js?:307:19)
    at SignatureV4.eval (webpack://backend/../node_modules/@aws-sdk/signature-v4-multi-region/node_modules/@aws-sdk/signature-v4/dist-es/SignatureV4.js?:50:30)
    at step (webpack://backend/../node_modules/tslib/tslib.es6.js?:130:23)
    at Object.eval [as next] (webpack://backend/../node_modules/tslib/tslib.es6.js?:111:53)
    at fulfilled (webpack://backend/../node_modules/tslib/tslib.es6.js?:101:58)

I'm using (what is currently) the latest javascript aws sdk, version 3.165.0. What am I missing here?