AWS Rekognition: Unable to get image metadata from S3. Check object key, region and/or access permissions.

What does this Amazon Rekognition error mean: Unable to get image metadata from S3.  Check object key, region and/or access permissions.

For me, this meant that I was calling

DetectFaces(Image => { S3Object => { Bucket => “bucket_name”, Name => “path_to_image” } }

with bucket_name belonging to an S3 bucket in a different region than the Rekognition endpoint that I was using.  It turns out, Rekognition can only access buckets in its region.  Copying the data to us-west-2 resolved the problem.