Passports are not magical booklets that effortlessly let you travel the world without a spot of bother. First, your passport is the property of the United States government, not yours, so it has to follow the rules laid by the US government to be valid. Some of these rules are surprising, but if not respected, will wreck a trip real quick. Second, the immigration laws of the countries you plan to visit involve specific and unexpected requirements from your passport that have to be met. Although the US passport is the fifth strongest in the world, there are still ways it can keep you from traveling. Here are four of them.

1. You don’t have enough blank pages.

If you’re one of those travelers who enjoy collecting stamps and visas in your passport, watch out. According to the Bureau of Consular Affairs, you need two to four blank visa pages in your passport or you may be denied entry at the border of your destination or even turned down from boarding the plane that will take you there. Of course, it depends on each country’s immigration rules, but why would you want to take a chance? Make sure you send an application to renew your passport when it looks like you’re running out of space for customs to stamp, stick, or staple your visa. If you travel a lot, we suggest that you order a 52-page passport at no extra charge.

2. Your passport expires in 6 months or less.

It’s simple, if your passport expires in 6 months or less, you may be denied to travel. It sounds bananas — what’s the use of the expiration date, then? — but it’s very much something you should keep in mind. Horror stories of people being turned away at the check-in counter on the day of their departure are all over the internet.

The Bureau of Consular Affairs has made a handy list (click on the country you are traveling to on the map) of passport validity requirements by country. It varies greatly, so don’t assume that because you’re traveling to Sweden — where a 6-month validity is recommended — it will be the same in Spain — where you need a 3-month validity beyond your date of departure.

Of course, you can apply for an emergency passport if you’re stuck and absolutely need to go abroad, but we suggest you just make a note in your calendar to replace your passport six months early — it’ll save you a lot of anxiety.

3. Your passport is invalid for one of these small reasons.

  • If your passport is not signed, it’s not valid. It only takes 2 seconds, so do it before a fussy border agent pings you and gives you some trouble.
  • If you’ve drawn/written on your passport. Buy a notebook and get creative, but leave your passport alone and keep it away from your kids and their crayons.
  • Make certain you don’t bend it so the chip isn’t busted and ensure that everything is legible — if your document ever got wet and made some entries hard to read, you’re taking a chance to be turned down and/or have your passport taken away. Torn pages and cover are also a big no-no.
  • Unauthorized stamps like the fancy ones you get at Machu Picchu, at the post office in Monte Carlo, or at Checkpoint Charlie in Berlin are nothing but souvenir stamps that may get your passport declared invalid. Passport stamps are cool, so this sucks, but if you may need to walk the line if you want to keep your travel document valid for as long as you can. Note that unofficial stamps can have you denied entry in some countries; all it takes is a zealous border agent.

4. You didn’t bring your passport to fly domestically.

If you’re accustomed to just bringing your driver’s license to fly within the United States, you’ll soon run into major trouble if you hail from certain states. Boarding federally-regulated commercial aircraft will soon require all American travelers to have an ID that meets the REAL ID Act requirements. If your state has not yet made the switch (check out this map to see if that’s the case), it has until October 10, 2018 to do so. After that, you better get yourself a passport or you won’t be going anywhere by airplane.