diff --git a/src/util/cout_message.h b/src/util/cout_message.h
index 483708cb21f..2c53b576896 100644
--- a/src/util/cout_message.h
+++ b/src/util/cout_message.h
@@ -29,6 +29,14 @@ class cerr_message_handlert:public stream_message_handlert
 class console_message_handlert : public message_handlert
 {
 public:
+  void print(unsigned, const xmlt &) override
+  {
+  }
+
+  void print(unsigned, const jsont &) override
+  {
+  }
+
   // level 4 and upwards go to cout, level 1-3 to cerr
   virtual void print(
     unsigned level,
@@ -57,6 +65,14 @@ class console_message_handlert : public message_handlert
 class gcc_message_handlert : public console_message_handlert
 {
 public:
+  void print(unsigned, const xmlt &) override
+  {
+  }
+
+  void print(unsigned, const jsont &) override
+  {
+  }
+
   // aims to imitate the messages gcc prints
   virtual void print(
     unsigned level,
diff --git a/src/util/message.h b/src/util/message.h
index 7daa5504586..752cd37eb86 100644
--- a/src/util/message.h
+++ b/src/util/message.h
@@ -30,25 +30,16 @@ class message_handlert
 
   virtual void print(unsigned level, const std::string &message)=0;
 
-  virtual void print(unsigned level, const xmlt &xml)
-  {
-    // no-op by default
-  }
+  virtual void print(unsigned level, const xmlt &xml) = 0;
 
-  virtual void print(unsigned level, const jsont &json)
-  {
-    // no-op by default
-  }
+  virtual void print(unsigned level, const jsont &json) = 0;
 
   virtual void print(
     unsigned level,
     const std::string &message,
     const source_locationt &location);
 
-  virtual void flush(unsigned level)
-  {
-    // no-op by default
-  }
+  virtual void flush(unsigned) = 0;
 
   virtual ~message_handlert()
   {
@@ -80,18 +71,30 @@ class message_handlert
 class null_message_handlert:public message_handlert
 {
 public:
-  virtual void print(unsigned level, const std::string &message)
+  void print(unsigned level, const std::string &message) override
   {
     message_handlert::print(level, message);
   }
 
-  virtual void print(
+  void print(unsigned, const xmlt &) override
+  {
+  }
+
+  void print(unsigned, const jsont &) override
+  {
+  }
+
+  void print(
     unsigned level,
     const std::string &message,
-    const source_locationt &)
+    const source_locationt &) override
   {
     print(level, message);
   }
+
+  void flush(unsigned) override
+  {
+  }
 };
 
 class stream_message_handlert:public message_handlert
@@ -101,7 +104,7 @@ class stream_message_handlert:public message_handlert
   {
   }
 
-  virtual void print(unsigned level, const std::string &message)
+  void print(unsigned level, const std::string &message) override
   {
     message_handlert::print(level, message);
 
@@ -109,7 +112,15 @@ class stream_message_handlert:public message_handlert
       out << message << '\n';
   }
 
-  virtual void flush(unsigned level)
+  void print(unsigned, const xmlt &) override
+  {
+  }
+
+  void print(unsigned, const jsont &) override
+  {
+  }
+
+  void flush(unsigned) override
   {
     out << std::flush;
   }