diff --git a/include/msgpack/adaptor/ext.hpp b/include/msgpack/adaptor/ext.hpp
index 84d67a6b..0fe836b4 100644
--- a/include/msgpack/adaptor/ext.hpp
+++ b/include/msgpack/adaptor/ext.hpp
@@ -183,7 +183,7 @@ private:
     friend struct msgpack::adaptor::object<msgpack::type::ext_ref>;
 };
 
-ext::ext(ext_ref const& x) {
+inline ext::ext(ext_ref const& x) {
     // size limit has aleady been checked at ext_ref's constructor
     m_data.reserve(x.size() + 1);