diff --git a/expat/lib/xmlparse.c b/expat/lib/xmlparse.c
--- a/expat/lib/xmlparse.c
+++ b/expat/lib/xmlparse.c
@@ -2357,7 +2357,13 @@ XML_GetCurrentByteIndex(XML_Parser parse
   if (parser->m_eventPtr)
     return (XML_Index)(parser->m_parseEndByteIndex
                        - (parser->m_parseEndPtr - parser->m_eventPtr));
+/* BEGIN MOZILLA CHANGE (fix XML_GetCurrentByteIndex) */
+#if 0
   return -1;
+#else
+  return parser->m_parseEndByteIndex;
+#endif
+/* END MOZILLA CHANGE */
 }
 
 int XMLCALL