diff --git a/libqpdf/qpdf/QPDF_Null.hh b/libqpdf/qpdf/QPDF_Null.hh
index 086a322..dffe026 100644
--- a/libqpdf/qpdf/QPDF_Null.hh
+++ b/libqpdf/qpdf/QPDF_Null.hh
@@ -7,7 +7,7 @@ class QPDF_Null: public QPDFObject
 {
   public:
     virtual ~QPDF_Null();
-    std::string unparse();
+    virtual std::string unparse();
 };
 
 #endif // __QPDF_NULL_HH__
diff --git a/libqpdf/qpdf/QPDF_Real.hh b/libqpdf/qpdf/QPDF_Real.hh
index 7c4f3ce..2895cee 100644
--- a/libqpdf/qpdf/QPDF_Real.hh
+++ b/libqpdf/qpdf/QPDF_Real.hh
@@ -9,7 +9,7 @@ class QPDF_Real: public QPDFObject
     QPDF_Real(std::string const& val);
     QPDF_Real(double value, int decimal_places = 0);
     virtual ~QPDF_Real();
-    std::string unparse();
+    virtual std::string unparse();
     std::string getVal();
 
   private: