diff --git a/rtti.cpp b/rtti.cpp
index acb0621..713ff66 100644
--- a/rtti.cpp
+++ b/rtti.cpp
@@ -5,7 +5,7 @@
 
 class Nail {
  public:
-  Nail(float length)
+  explicit Nail(float length)
   : length(length) {}
 
   float getLength() {