Error while using lxc-attach : Failed to open terminal multiplexer device

When I’m trying to use lxc-attach command this error prints out lxc_terminal_create_native: 927 Not a directory - Failed to open terminal multiplexer device. I don’t know how to fix this. My environment is that I install lxc on openwrt and the lxc container is another openwrt. So the setup is like openwrt on top of openwrt and then im trying to use lxc-attach this is the error message. Tho its working fine and i can access the another openwrt but im just bothered with the error. What should I do to fix this error? Your reply is greatly appreciated! Thank you!