diff --git a/graph.hpp b/graph.hpp
index cca0ea8..afe1f97 100644
--- a/graph.hpp
+++ b/graph.hpp
@@ -131,7 +131,7 @@ public:
 
       The number of nodes in the graph currently cannot be changed. You can only add edges between the existing nodes.
    **/
-   Graph(NodeId const num_nodes);
+   explicit Graph(NodeId const num_nodes);
 
    /** @return The number of nodes in the graph. **/
    NodeId num_nodes() const;