C# แปลงตัวเลขเป็นข้อความภาษาอังกฤษแล้วเก็บค่าลงใน List นอก method ไม่ได้ครับ

 public List<String> NumberToWordEN(int number)
        {
           // List<string> ttsEN = new List<string>();   // ตรงนี้ประกาศนอก method แล้วค่าผิดพลาดข้อความซ้ำกัน

            var unitsMap = new[] { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };
            var tensMap = new[] { "zero", "ten", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety" };


            if (number == 0)
            {
                ttsEN.Add("zero");
                return ttsEN;
            }
            string words = "";

            if ((number / 1000000) > 0)
            {
                words += NumberToWordEN(number / 1000000) + " million ";

                ttsEN.Add(unitsMap[(number / 1000000)]);
                ttsEN.Add("million");

                number %= 1000000;
            }

            if ((number / 1000) > 0)
            {
                words += NumberToWordEN(number / 1000) + " thousand ";

                ttsEN.Add(unitsMap[(number / 1000)]);
                ttsEN.Add("thousand");

                number %= 1000;
            }

            if ((number / 100) > 0)
            {
                words += NumberToWordEN(number / 100) + " hundred ";

                ttsEN.Add(unitsMap[(number / 100)]);
                ttsEN.Add("hundred");

                number %= 100;
            }

            if (number > 0)
            {
                if (words != "")
                {
                    words += "and ";
                    ttsEN.Add("and");
                }

                if (number < 20)
                {
                    words += unitsMap[number];
                    ttsEN.Add(unitsMap[number]);   // ถ้า comment ตรงนี้จะได้ค่าถูกต้องแต่ไม่มี fifteen หลัง and
                }
                else
                {
                    words += tensMap[number / 10];
                    ttsEN.Add(tensMap[number / 10]); 

                    if ((number % 10) > 0)
                    {
                        words += "-" + unitsMap[number % 10];
                        ttsEN.Add(unitsMap[number % 10]);
                    }
                }

            }
            return ttsEN;

        }