AVPlayerItem externalMetadata unrecognized selector

This piece of code is crashing on iOS 12 with unrecognised selector exception:

[AVPlayerItem externalMetadata]: unrecognized selector sent to instance

However, this code is not crashing on iOS 13.

Apple documentation states that AVPlayerItem has externalMetadata getter and setter since iOS 12.

    let item = AVPlayerItem(url: videoURL)

    let mutableMetadataItem = AVMutableMetadataItem()
    mutableMetadataItem.identifier = AVMetadataIdentifier.commonIdentifierDescription
    mutableMetadataItem.value = 1
    item.externalMetadata.append(mutableMetadataItem)

Could you please help me to understand why is that happening?