Spurious error about existing bucket

[BenLangmead]

Scenario: I have a bucket blah and my EMR job is set to output to a subdirectory of blah. When I try to run Rail starting from json:

cat hg19.json | python src/dooplicity/emr_runner.py --region us-east-1

I get an error:

RuntimeError: Bucket `blah` already exists on S3. Change affected output directories in job flow and try again. The more distinctive the name chosen, the less likely it's already a bucket on S3. It may be easier to create a bucket first using the web interface and use its name + a subdirectory as the output directory.

I don't know why the bucket already existing should pose a problem here.