diff --git a/graph.cpp b/graph.cpp
index 5041d36..16b5710 100644
--- a/graph.cpp
+++ b/graph.cpp
@@ -23,6 +23,8 @@
  */
 #include <stdexcept>
 
+#include <cassert>
+
 // Anonymous name spaces may be used to show the reader
 // that a function will only be used in the current file.
 namespace
diff --git a/graph.hpp b/graph.hpp
index 5622327..8b71861 100644
--- a/graph.hpp
+++ b/graph.hpp
@@ -40,6 +40,7 @@
  */
 #include <vector>
 
+#include <cassert>
 /**
  * Namespaces can be used in order to make sure different modules,
  * possibly implemented by different people don't have classes/functions/...
@@ -238,18 +239,21 @@ Node & Graph::node(NodeId const id)
 inline
 NodeId Graph::matched_neighbor(NodeId const id) const
 {
+   assert(id <= num_nodes());
    return _nodes[id].matched_neighbor;
 }
 
 inline
 NodeId Graph::ear_or_root_neighbor(const ED::NodeId id) const
 {
+   assert(id <= num_nodes());
    return _nodes[id].ear_or_root_neighbor;
 }
 
 inline
 NodeId Graph::root_of_ear_component(const ED::NodeId id) const
 {
+   assert(id <= num_nodes());
    return _nodes[id].root_of_ear_component;
 }