Index: /Users/quelle/Documents/workspace/jruby_1_0_1/src/org/jruby/util/Pack.java
===================================================================
--- /Users/quelle/Documents/workspace/jruby_1_0_1/src/org/jruby/util/Pack.java	(revision 4620)
+++ /Users/quelle/Documents/workspace/jruby_1_0_1/src/org/jruby/util/Pack.java	(working copy)
@@ -35,6 +35,7 @@
 package org.jruby.util;
 
 import java.math.BigInteger;
+import java.nio.BufferUnderflowException;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.nio.CharBuffer;
@@ -855,12 +856,40 @@
                     break;
                 case 'U' :
                     {
+                        ByteBuffer buffer;
                         if (occurrences == IS_STAR || occurrences > encode.remaining()) {
-                            occurrences = encode.remaining();
+                            byte[] toUnpack = new byte[encode.remaining()];
+                            encode.get(toUnpack);
+                            buffer = ByteBuffer.wrap(toUnpack);
+                        } else {
+                            byte[] toUnpack = new byte[occurrences*4];
+                            int pos = 0;
+                            try {
+                                while (--occurrences >= 0 && encode.hasRemaining()) {
+                                    byte b = encode.get();
+                                    if (b >= (byte)0x00 && b <= (byte)0x7F) {
+                                        toUnpack[pos++] = b;
+                                    } else if (b >= (byte)0xC2 && b <= (byte)0xDF) {
+                                        toUnpack[pos++] = b;
+                                        toUnpack[pos++] = encode.get();
+                                    } else if (b >= (byte)0xE0 && b <= (byte)0xEF) {
+                                        toUnpack[pos++] = b;
+                                        encode.get(toUnpack, pos, 2);
+                                        pos += 2;
+                                    } else if (b >= (byte)0xF0 && b <= (byte)0xF4) {
+                                        toUnpack[pos++] = b;
+                                        encode.get(toUnpack, pos, 3);
+                                        pos += 3;
+                                    } else {
+                                        throw runtime.newArgumentError("malformed UTF-8 character");
+                                    }
+                                }
+                            } catch (BufferUnderflowException e) {
+                                throw runtime.newArgumentError("malformed UTF-8 character");
+                            }
+                            buffer = ByteBuffer.wrap(toUnpack, 0, pos);
                         }
                         //get the correct substring
-                        byte[] toUnpack = new byte[occurrences];
-                        encode.get(toUnpack);
                         CharBuffer lUtf8 = null;
                         try {
                             Charset utf8 = Charset.forName("UTF-8");
@@ -867,7 +896,6 @@
                             CharsetDecoder utf8Decoder = utf8.newDecoder();
                             utf8Decoder.onMalformedInput(CodingErrorAction.REPORT);
                             utf8Decoder.onUnmappableCharacter(CodingErrorAction.REPORT);
-                            ByteBuffer buffer = ByteBuffer.wrap(toUnpack);
 
                             lUtf8 = utf8Decoder.decode(buffer);
                         } catch (CharacterCodingException cce) {
@@ -874,7 +902,7 @@
                             // invalid incoming bytes; fail to encode.
                             throw runtime.newArgumentError("malformed UTF-8 character");
                         }
-                        while (occurrences-- > 0 && lUtf8.hasRemaining()) {
+                        while (lUtf8.hasRemaining()) {
                             long lCurChar = lUtf8.get();
                             result.append(runtime.newFixnum(lCurChar));
                         }