If you’re under 13 or have a twin, it might not be a good idea to use the Face ID unlocking system on Apple’s new iPhone X.

The U.S. technology giant released security guidelines on Wednesday about Face ID – the iPhone X feature that allows users to unlock the device just by looking at it. In the document, Apple said that the probability of a random person unlocking an iPhone X with Face ID is 1 in 1,000,000 versus 1 in 50,000 for its previous fingerprint sensor.

But it could have a problem with twins or under 13s.

“The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you’re concerned about this, we recommend using a passcode to authenticate,” Apple said in the guidelines.

Face ID uses the TrueDepth selfie camera on the $999 iPhone X to analyze over 30,000 “invisible dots” on the face to create a 3-D image. Apple’s artificial intelligence network does the analysis and matches it to your face to unlock the phone.

Apple says that the system works even when someone is wearing a hat or sunglasses for example. One of the interesting aspects of Face ID is how it adapts to you as your face changes. In an accompanying white paper, Apple describes how Face ID “augments its stored mathematical representation over time.” This means that the stored image that Apple has of your face changes as your face develops.

Face ID may capture new images of your face upon successful unlock attempts. This helps develop the stored picture of the face. Also, if you fail to unlock the phone with your face, but follow it up with a passcode, Apple may still store that image of your face because it could be used to augment the stored image on the iPhone X.

The company went to lengths to explain the safeguards in place so Face ID can’t be spoofed, one of the concerns raised about the system, after fingerprint sensors have regularly been found to be flawed.

Apple said that the Face ID is encrypted, secured, and never backed up to Apple’s cloud computing service iCloud or anywhere else.

“Face ID automatically adapts to changes in your appearance, and carefully safeguards the privacy and security of your biometric data,” Apple said in the white paper.