Tmux and open: "The window server could not be contacted...."

While running tmux, attempts to open a file/document, as in:
open index.html
gives this error:
The window server could not be contacted. open must be run with a user logged in at the console, either as that user or as root.
Works fine if not in a tmux window. This post provides a detailed response and then suggests tmux kill-server which is certainly not what I want.

Any suggestions? Thanks