diff --git a/exprtk.hpp b/exprtk.hpp
index b4530ba..11f4507 100644
--- a/exprtk.hpp
+++ b/exprtk.hpp
@@ -5849,7 +5849,7 @@ namespace exprtk
                   return const_value_ptr(0);
             }
 
-            std::size_t vector_size() const
+            std::size_t vector_size() const exprtk_override
             {
                return size_;
             }