Description of problem:
When I attempt to run Docker exec on a running container I get the following error:

Error response from daemon: Unsupported: Exec is not supported by the lxc driver


Version-Release number of selected component (if applicable):
# uname -a
Linux 2.6.32-504.el6.x86_64 #1 SMP Tue Oct 14 01:47:47 PDT 2014 x86_64 x86_64 x86_64 GNU/Linux

# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.6 (Santiago)

# docker version
Client version: 1.3.1
Client API version: 1.15
Go version (client): go1.3.3
Git commit (client): 4e9bbfa
OS/Arch (client): linux/amd64
Server version: 1.3.1
Server API version: 1.15
Go version (server): go1.3.3
Git commit (server): 4e9bbfa


How reproducible: 100%


Steps to Reproduce:
1. $ docker exec -it my_docker_image bash

Actual results:
Error response from daemon: Unsupported: Exec is not supported by the lxc driver

Expected results:
The expected result is that I get a bash prompt.

Additional info: