diff --git a/lib/cp949.dart b/lib/cp949.dart
index 53e0380..6afec98 100644
--- a/lib/cp949.dart
+++ b/lib/cp949.dart
@@ -1,19 +1,20 @@
 import 'src/code_map.dart' show cp949ToUnicodeCodeMap, unicodeToCp949CodeMap;
 
 List<int> toUnicodes(final List<int> codeUnits) {
-  final unicodes = List<int>(codeUnits.length);
+  final List<int> unicodes = new List<int>.filled(codeUnits.length, 0);
+  // new List(codeUnits.length);
   var j = 0;
   for (var i = 0; i < codeUnits.length; j++) {
     // When 1 byte becomes 1 cp949 code
     if (0x00 <= codeUnits[i] && codeUnits[i] <= 0x7F) {
-      unicodes[j] = cp949ToUnicodeCodeMap[codeUnits[i]];
+      unicodes[j] = cp949ToUnicodeCodeMap[codeUnits[i]]!;
       i = i + 1;
     } else {
       // When 2 bytes become 1 cp949 code
       final cp949Code = (codeUnits[i] << 8) + codeUnits[i + 1];
       i = i + 2;
       if (0x8141 <= cp949Code && cp949Code <= 0xFDFE) {
-        unicodes[j] = cp949ToUnicodeCodeMap[cp949Code];
+        unicodes[j] = cp949ToUnicodeCodeMap[cp949Code]!;
       } else {
         throw FormatException(
             'Invalid code unit of CP949. It has to be (>=0x00 <=0x7F) || (>=0x8141 <=0xFDFE).');
@@ -34,10 +35,10 @@ String decodeString(final String brokenString) {
 /// Dart does not support non-unicode encoding.
 /// Thus, the return value has to be raw byte array of CP949.
 List<int> encode(final String str) {
-  final cp949codeUnits = List<int>(str.codeUnits.length * 2);
+  final cp949codeUnits = List<int>.filled(str.codeUnits.length * 2, 0);
   var i = 0;
   for (final unicode in str.codeUnits) {
-    final cp949Code = unicodeToCp949CodeMap[unicode];
+    final cp949Code = unicodeToCp949CodeMap[unicode]!;
     final firstByte = cp949Code >> 8;
     final secondByte = cp949Code - (firstByte << 8);
     if (firstByte != 0) {