diff --git a/src/vtk/Base64Writer.h b/src/vtk/Base64Writer.h
index 1445da4a6c6068346e8956c6d42223e2ae662021..a619015592708f4cccb249384143d515fa13471f 100644
--- a/src/vtk/Base64Writer.h
+++ b/src/vtk/Base64Writer.h
@@ -75,7 +75,7 @@ public:
    void toStream( std::ostream& os );
 
 private:
-   void encodeblock( const std::array< unsigned char, 3 > in,  std::array< unsigned char, 4 > out, int len )
+   void encodeblock( const std::array< unsigned char, 3 > in,  std::array< unsigned char, 4 > & out, int len )
    {
       std::array< unsigned char, 65 > cb64 { "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" };