Converting to Arabic numbers using tuples

ArabicTuples.swift

import UIKit

func toArabic(romanNumerals: String) -> Int? {
    let values = [("M", 1000), ("CM", 900), ("D", 500), ("CD", 400), ("C",100), ("XC", 90), ("L",50), ("XL",40), ("X",10), ("IX", 9), ("V",5),("IV",4), ("I",1)]

    var romanNumerals = romanNumerals
    var result = 0

    for (romanChars, arabicValue) in values {
        let range = romanChars.startIndex ..< romanChars.endIndex

        while romanNumerals.hasPrefix(romanChars) {
            result = result + arabicValue

            romanNumerals.removeSubrange(range)
        }
    }
    
    guard (romanNumerals.isEmpty) else {
        return nil
    }

    return result
}

toArabic(romanNumerals: "MCM") // Some 1,900
toArabic(romanNumerals: "MCMZ") // nil
toArabic(romanNumerals: "ROB") // nil

Thanks for the feedback and corrections.