diff --git a/apps/node.cpp b/apps/node.cpp
index cd26dc8d9..332d87e14 100644
--- a/apps/node.cpp
+++ b/apps/node.cpp
@@ -1,6 +1,6 @@
 #include "node.h"
 
-int Node::numberOfChildren() {
+int Node::numberOfChildren() const {
   return m_numberOfChildren;
 }
 
@@ -8,6 +8,6 @@ const char * Node::label() const {
   return m_label;
 }
 
-bool Node::isNull() {
+bool Node::isNull() const {
   return (m_label == nullptr);
 }
diff --git a/apps/node.h b/apps/node.h
index d604de226..0c6394c3b 100644
--- a/apps/node.h
+++ b/apps/node.h
@@ -10,8 +10,8 @@ public:
   };
   virtual const Node * children(int index) const = 0;
   const char * label() const;
-  int numberOfChildren();
-  bool isNull();
+  int numberOfChildren() const;
+  bool isNull() const;
 protected:
   const char * m_label;
   int m_numberOfChildren;