为什么两个不重叠的CharacterSets的交集是非空的？

我试图使用CharacterSet来检查用户输入字符串是否包含任何非十进制数字字符.我使用CharacterSet.decimalDigits并与用户输入的交集.如果此交叉点为空,则可能意味着用户未输入有效输入.但十字路口并非空洞.

let digits = CharacterSet.decimalDigits
let letters = CharacterSet(charactersIn: "abcd") // never prints

let intersection = digits.intersection(letters)
for c in "abcd".characters {
    if intersection.contains(UnicodeScalar(String(c))!) {
        print("contains \(c)") // never prints
    }
}

for i in 0...9 {
    if intersection.contains(UnicodeScalar(String(i))!) {
        print("contains \(i)")
    }
}

print("intersection is empty: \(intersection.isEmpty)") // prints false

for i in 0x0000...0xFFFF {
    guard let c = UnicodeScalar(i) else {
        continue
    }
    if intersection.contains(c) {
        print("contains \(c)")
    }
}