Donald Trump has now explained his plan to force Mexico to pay to build a wall along the border with the United States: He would bar undocumented immigrants in the United States from remitting money to Mexico. That threat to the Mexican economy, Trump says, would force the government to pay up.

It isn’t a credible threat, however, according to experts on the intricate network of software, firms and people that moves money across international borders.

If it’s a challenge for authorities to prevent people from slipping across borders illegally, it’s even more difficult with currency. Already, many financial institutions are finding they are unable to shut terrorists and criminals out of the international money transfer system — a smaller task than preventing the nearly 11 million undocumented immigrants in the United States from remitting what they’ve earned here.

Then there’s the possibility of smuggling cash. Experts point out that, by monitoring money transfers above ground, Trump’s plan might just create a black market for remittances that’s even more difficult to track.