gsutil cp -R dir gs://my-bucket doesn't copy subdirectories

I just tried copying a directory containing several files and directories using:

gsutil -m cp -r dir gs://my-bucket

It copied only the top-level files from dir to the bucket.

I'm using:

gsutil version: 4.27
checksum: 522455e2d24593ff3a2d3d237eefde57 (OK)
boto version: 2.47.0
python version: 2.7.6 (default, Oct 26 2016, 20:30:19) [GCC 4.8.4]
OS: Linux 4.4.0-83-generic
multiprocessing available: True
using cloud sdk: False
pass cloud sdk credentials to gsutil: False
config path(s): /usr/local/google/home/mfschwartz/.boto_prod_oauth
gsutil path: /usr/local/google/home/mfschwartz/gsutil/gsutil
compiled crcmod: True
installed via package manager: False
editable install: False