If you’re planning on watching the total solar eclipse, be sure to prep.

Most people are warned, especially as children, to never look directly at the sun. But when there’s an event that literally requires you to do so in order to experience it, special eyewear is a must.

Related: Eclipse Day Timeline: Exactly What Will Happen on August 21

People who are planning on watching the Great American Eclipse on August 21 have been buying up plenty of sun-watching eyewear in preparation for the solar event. However, according to The Verge, Amazon has been cracking down on vendors who are potentially selling fake or defective gear to customers.

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The company has issued refunds to people who have already bought special glasses and eyewear from suspected fake vendors, issuing a statement to statement to Oregon broadcaster KGW:

“Safety is among our highest priorities. Out of an abundance of caution, we have proactively reached out to customers and provided refunds for eclipse glasses that may not comply with industry standards. We want customers to buy with confidence anytime they make a purchase on Amazon.com and eclipse glasses sold on Amazon.com are required to comply with the relevant ISO standard.”

Additionally, Amazon has already removed certain pages that sell suspected fake glasses. Products who have been unable to verify themselves as a quality manufacturer include the “MASCOTKING Solar Eclipse Glasses 2017 - CE and ISO Certified Safe Shades for Direct Sun Viewing — Eye Protection,” according to The Verge.

Only five manufacturers are recognized by both NASA and American Astronomical Society (AAS) for meeting ISO standards for viewing the eclipse, including Rainbow Symphony, American Paper Optics, Thousand Oaks Optical, TSE 17 and Baader Planetarium.

If you are planning to view the eclipse with a pair of special glasses, the AAS issued a warning on their website, stating, “as long as you can trace your filters to a reputable vendor or other reliable source, and as long as they have the ISO logo and a statement attesting to their ISO 12312-2 compliance, you should have nothing to worry about.” There is also a list of vendors on their website.

Of course, if you’re looking to go the DIY route, two index cards with pinholes in them should also work just fine. There are lots of tutorials on how to make safe, DIY eclipse viewers online.