diff --git a/lib/java/src/org/apache/thrift/TUnion.java b/lib/java/src/org/apache/thrift/TUnion.java
index 7e4f0af..353e11c 100644
--- a/lib/java/src/org/apache/thrift/TUnion.java
+++ b/lib/java/src/org/apache/thrift/TUnion.java
@@ -212,7 +212,7 @@
       if (i != 0) {
         sb.append(" ");
       }
-      String digit = Integer.toHexString(bytes[i]);
+      String digit = Integer.toHexString(bytes[i] & 0xFF);
       sb.append(digit.length() > 1 ? digit : "0" + digit);
     }
     if (bytes.length > 128) {