A Facebook executive says the social network will begin using paper postcards to try and stop foreign meddlers—for instance, Russian agents—from buying ads designed to influence U.S. elections. Before the 2016 Presidential election, nearly 10 million Americans saw ads that were linked to Russia’s elaborate propaganda campaign.
To try and prevent that from happening again, Facebook says it will mail ad buyers a postcard bearing a code. According to Katie Harbath, Facebook’s global director of policy programs, “You will have to use that code to prove you are in the United States.” Harbath’s comments came at a weekend conference of state government officials and were earlier reported by Reuters.
However, according to Harbath, the program will only be applied to ads that mention specific candidates. That might limit its intended impact, since social media propaganda appears to have frequently focused on broader issues of identity or ideology, with the aim of stirring discontent rather than supporting a particular candidate.
Get Data Sheet, Fortune’s technology newsletter.
Another loophole is that the plan doesn’t seem to address ad buyers’ citizenship status. While permanent residents with green cards can donate to U.S. political campaigns, foreign nationals are generally prohibited from buying ads or otherwise spending money to influence any U.S. political campaign—even if they have a U.S. address.
Facebook does say this is one move in a broader effort to clamp down on deceptive use of its platform. Harbath didn’t say exactly when the program would become active, but did say it will be in place ahead of November’s midterm Congressional elections.