Why does a civ say: Can't Trade ?

[RP21]

On the technology screen of the Foreign Advisor, I sometimes see the phrase "Can't Trade" as opposed to "Won't Trade". If I bring-up the diplo screen for one of the civs, the "can't trade" techs are not listed. ("won't trade" techs are listed in red.) Does anyone know what the distinction is? What prevents a tech from being traded? In my current game, most of the other civs have Fascism and Industrialization but they all say they can't trade them.

[rjmatsleepers]

I assumed that the "can't trade" technologies are ones for which you don't have the required precursor techs. But perhaps it's more complicated than that since new technology trade options seem to appear on the turn after a round of trading.

RJM at Sleeper's

[trev]

The Can't Trades are the techs for which you did not have the prerequisites for currently, or if in same turn as you have been trading techs, did not have the prerequisites prior to the trades.