The US government is weighing a proposal to ask foreign visitors to disclose their social media accounts when entering or leaving the country.

According to the Federal Register, the proposal from the US Department of Homeland Security would add the following line to arrival and departure forms for non-citizens, and on electronic forms for travelers entering on a visa waiver:

Please enter information associated with your online presence—Provider/Platform—Social media identifier

The department, which is accepting written public comment on the proposal through Aug. 22, believes that collecting social media information will help it detect “possible nefarious activity and connections.” But it’s unclear how effective it will be detecting potential terrorists. To start with, the field is entirely optional, so foreigners can choose to withhold their social media information from travel forms.

Moreover, there’s no way of ensuring that anyone choosing to answer would answer truthfully, making the exercise about as helpful as that old line of questioning for travelers about whether they’d packed their own bags or had been asked by someone to carry anything onto the flight for them.

Meanwhile, the information that’s publicly posted under travelers’ accounts might not reveal criminal intentions—and it seems unlikely social media providers will hand over users’ private information or messages, especially in light of the recent showdown between Apple and the FBI.

The US government came under criticism after a mass shooting in San Bernardino, California, because one of the assailants, Tashfeen Malik, had obviously things went wrong” in the visa process.