diff --git a/include/sdf/Plugin.hh b/include/sdf/Plugin.hh
index 05fbc8a52..97b04dee3 100644
--- a/include/sdf/Plugin.hh
+++ b/include/sdf/Plugin.hh
@@ -189,7 +189,7 @@ namespace sdf
     }
 
     /// \brief Private data pointer.
-    std::unique_ptr<sdf::PluginPrivate> dataPtr;
+    private: std::unique_ptr<sdf::PluginPrivate> dataPtr;
   };
 
   /// \brief A vector of Plugin.