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If your passport is packed with entry stamps and visas, there’s some paperwork you should fill out ASAP.

On December 31, the Department of State will stop letting travelers add extra pages to their passports, said Brenda Sprague, deputy assistant secretary for passport services. Run out of space after that, and you’ll have to get a whole new book for 2016.

The change is all about security: the State Department plans to release a new passport design in late 2016, and one of the many new features is numbered visa pages. With the lack of numbers in the current design, there’s a risk of “page substitution,” Sprague said. “Or, people tearing pages out so you don’t know where they’ve been.”

Extra pages cost $82 for adults, while a renewal is $110 (with a $60 fee for expediting the process). Should you renew, you can request a 52-page passport, as opposed to the standard 28-page book, at no extra charge. But if you need a visa that’s in the old passport, you’ll have to travel with both.