Open Borders option not available

I'm playing Endless Space (with Disharmony), and I'm slightly confused by the diplomacy mechanics. I've made peace with another faction (Horatio, if it matters; I'm Sophons myself). I'd like to offer them to open borders, but when the option is greyed out in the menu, and if I hover over it, I get

The terms you request cannot be fulfilled.

Now I know they also need to have researched that diplomacy option, but I know they've got the Relativistic Markets tech, because I gave it to them.

What other requirements are there to open borders with another faction?

I think that they need to discover at least one of your systems before you can get open borders with them. I had the same problem and could only open borders when one of their scouts entered my system through a bordering wormhole.

Source: personal experience