From the traceback it seems to be the S3 backend branch (/home/customer/LAFS_source on the storage server), not the cloud backend branch (/home/zooko/playground/tahoe-lafs/cloud-backend on the gateway), that is relevant. In any case the S3Bucket class from which the error is raised appears in both with few changes.
It seems that S3Bucket.get_object returned a Deferred for None (rather than a Deferred byte string), which is confusing. I think it can only happen if txaws.s3.client.S3Client.get_object returns a Deferred for None.
I looked briefly at the txaws code and it seems as though the return value comes from the client returned by twisted.web.client.HTTPClientFactory, but I can't tell whether that is giving None or whether the data is getting lost somewhere in txaws, or (less likely) in our error handling code.
