centos - docker exec -it returns "cannot enable tty mode on non tty input" -

docker exec -it command returns following error "cannot enable tty mode on non tty input"

level="fatal" msg="cannot enable tty mode on non tty input"  

i running docker(1.4.1) on centos box 6.6. trying execute following command docker exec -it containername /bin/bash getting following error

level="fatal" msg="cannot enable tty mode on non tty input"  

running docker exec -i instead of docker exec -it fixed issue. indeed, script launched crontab isn't terminal.

as reminder:

usage: docker exec [options] container command [arg...]  run command in running container    -i, --interactive=false    keep stdin open if not attached     -t, --tty=false            allocate pseudo-tty