Index: C:/workspace/jruby_101/src/org/jruby/util/Pack.java
===================================================================
--- C:/workspace/jruby_101/src/org/jruby/util/Pack.java	(revision 4615)
+++ C:/workspace/jruby_101/src/org/jruby/util/Pack.java	(working copy)
@@ -34,6 +34,9 @@
  ***** END LICENSE BLOCK *****/
 package org.jruby.util;
 
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
 import java.math.BigInteger;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
@@ -855,29 +858,20 @@
                     break;
                 case 'U' :
                     {
-                        if (occurrences == IS_STAR || occurrences > encode.remaining()) {
-                            occurrences = encode.remaining();
-                        }
-                        //get the correct substring
-                        byte[] toUnpack = new byte[occurrences];
-                        encode.get(toUnpack);
-                        CharBuffer lUtf8 = null;
+                        InputStreamReader reader = new InputStreamReader(new ByteArrayInputStream(
+                                encodedString.unsafeBytes()), Charset.forName("UTF-8"));
                         try {
-                            Charset utf8 = Charset.forName("UTF-8");
-                            CharsetDecoder utf8Decoder = utf8.newDecoder();
-                            utf8Decoder.onMalformedInput(CodingErrorAction.REPORT);
-                            utf8Decoder.onUnmappableCharacter(CodingErrorAction.REPORT);
-                            ByteBuffer buffer = ByteBuffer.wrap(toUnpack);
-
-                            lUtf8 = utf8Decoder.decode(buffer);
-                        } catch (CharacterCodingException cce) {
-                            // invalid incoming bytes; fail to encode.
+                            int c;
+                            if (occurrences == IS_STAR) {
+                                while ((c = reader.read()) != -1)
+                                    result.append(runtime.newFixnum(c));
+                            } else {
+                                while (occurrences-- > 0 && (c = reader.read()) != -1)
+                                    result.append(runtime.newFixnum(c));
+                            }
+                        } catch (IOException e) {
                             throw runtime.newArgumentError("malformed UTF-8 character");
                         }
-                        while (occurrences-- > 0 && lUtf8.hasRemaining()) {
-                            long lCurChar = lUtf8.get();
-                            result.append(runtime.newFixnum(lCurChar));
-                        }
                     }
                     break;
                  case 'X':