xsel and xclip hang

My objective is to make a text on my remote machine (CentOS 7.2) available to seamlessly paste on my local machine (OS X 10.12.2) with the standard ⌘V shortcut. My setup connects to the remote machine with ssh -Y and then attaches to tmux (or creates a new session if non-existent). When I run either echo "test" | xsel -ib or echo "test" | xclip it hangs. The $DISPLAY variable is localhost:10.0. If I exit tmux the $DISPLAY variable seems to be null and I get a can't open display error.