diff --git a/libs/xml/xmlparser.h b/libs/xml/xmlparser.h
index ecc7a043..b11d355b 100644
--- a/libs/xml/xmlparser.h
+++ b/libs/xml/xmlparser.h
@@ -79,7 +79,7 @@ class Formatted
 {
 public:
 const char* m_format;
-va_list m_arguments;
+mutable va_list m_arguments;
 Formatted( const char* format, ... )
 	: m_format( format ){
 	va_start( m_arguments, format );