diff --git a/include/aoc/Vertex.hpp b/include/aoc/Vertex.hpp
index 35f1b20..f696a03 100644
--- a/include/aoc/Vertex.hpp
+++ b/include/aoc/Vertex.hpp
@@ -23,10 +23,10 @@ class Vertex
 {
 public:
     Vertex(const std::string id);
-    std::string getId() const;
+    const std::string getId() const;
     const std::vector<std::weak_ptr<Vertex>>& getNeighbors() const;
     void addNeighbor(const std::shared_ptr<Vertex>& neighbor);
 private:
-    std::string id_;
+    const std::string id_;
     std::vector<std::weak_ptr<Vertex>> neighbors_;
 };
diff --git a/src/Vertex.cpp b/src/Vertex.cpp
index c6f31b7..5b52df0 100644
--- a/src/Vertex.cpp
+++ b/src/Vertex.cpp
@@ -22,7 +22,7 @@ Vertex::Vertex(const std::string id)
 {
 }
 
-std::string Vertex::getId() const
+const std::string Vertex::getId() const
 {
     return id_;
 }