Index: test/testPackUnpack.rb
===================================================================
--- test/testPackUnpack.rb	(revision 4841)
+++ test/testPackUnpack.rb	(working copy)
@@ -52,3 +52,6 @@
 test_exception(TypeError){ [-1, 0, 1, 128].pack("m") }
 test_equal(["testingT"], "testing=54".unpack("M*"))
 test_equal(["12345"], "12345".unpack("M*"))
+
+test_exception(ArgumentError){ "\205".unpack("U") }
+test_equal([228, 228], "\303\244\303\244\303\244".unpack("U2"))
Index: src/org/jruby/util/Pack.java
===================================================================
--- src/org/jruby/util/Pack.java	(revision 4841)
+++ src/org/jruby/util/Pack.java	(working copy)
@@ -905,7 +905,8 @@
                             occurrences = encode.remaining();
                         }
                         //get the correct substring
-                        byte[] toUnpack = new byte[occurrences];
+                        final int nBytes = Math.min(occurrences * 4, encode.remaining());
+                        byte[] toUnpack = new byte[nBytes];
                         encode.get(toUnpack);
                         CharBuffer lUtf8 = null;
                         try {