Index: src/org/jruby/util/Pack.java
===================================================================
--- src/org/jruby/util/Pack.java	(revision 8406)
+++ src/org/jruby/util/Pack.java	(working copy)
@@ -899,21 +899,21 @@
                         
                         // obtain a
                         s = safeGet(encode);
-                        while (((a = b64_xtable[s]) == -1) && encode.hasRemaining()) {
+                        while (((a = b64_xtable[s & 0xff]) == -1) && encode.hasRemaining()) {
                             s = safeGet(encode);
                         }
                         if (a == -1) break;
                         
                         // obtain b
                         s = safeGet(encode);
-                        while (((b = b64_xtable[s]) == -1) && encode.hasRemaining()) {
+                        while (((b = b64_xtable[s & 0xff]) == -1) && encode.hasRemaining()) {
                             s = safeGet(encode);
                         }
                         if (b == -1) break;
                         
                         // obtain c
                         s = safeGet(encode);
-                        while (((c = b64_xtable[s]) == -1) && encode.hasRemaining()) {
+                        while (((c = b64_xtable[s & 0xff]) == -1) && encode.hasRemaining()) {
                             if (s == '=') break;
                             s = safeGet(encode);
                         }
@@ -926,7 +926,7 @@
                         
                         // obtain d
                         s = safeGet(encode);
-                        while (((d = b64_xtable[s]) == -1) && encode.hasRemaining()) {
+                        while (((d = b64_xtable[s & 0xff]) == -1) && encode.hasRemaining()) {
                             if (s == '=') break;
                             s = safeGet(encode);
                         }