s3fs: Unable to put a local __init__.py on a S3 location

Trying to run the following

import s3fs
s3 = s3fs.S3FileSystem()
s3.put("./foo/__init__.py", "mybucket/__init__.py")

yields the error:

ClientError: An error occurred (MalformedXML) when calling the CompleteMultipartUpload operation: The XML you provided was not well-formed or did not validate against our published schema

Is it a bug or am I missing something?