diff --git a/erb/cpp03_define_map.hpp.erb b/erb/cpp03_define_map.hpp.erb
index 61a44660..e33e7785 100644
--- a/erb/cpp03_define_map.hpp.erb
+++ b/erb/cpp03_define_map.hpp.erb
@@ -20,6 +20,10 @@
 
 #if defined(MSGPACK_USE_BOOST)
 
+// BOOST_PP_VARIADICS is defined in boost/preprocessor/config/config.hpp
+// http://www.boost.org/libs/preprocessor/doc/ref/variadics.html
+// However, supporting compiler detection is not complete. msgpack-c requires
+// variadic macro arguments support. So BOOST_PP_VARIADICS is defined here explicitly.
 #define BOOST_PP_VARIADICS
 #include <boost/preprocessor.hpp>
 
diff --git a/include/msgpack/adaptor/detail/cpp03_define_map.hpp b/include/msgpack/adaptor/detail/cpp03_define_map.hpp
index 42db9839..d0ebc9ac 100644
--- a/include/msgpack/adaptor/detail/cpp03_define_map.hpp
+++ b/include/msgpack/adaptor/detail/cpp03_define_map.hpp
@@ -20,6 +20,10 @@
 
 #if defined(MSGPACK_USE_BOOST)
 
+// BOOST_PP_VARIADICS is defined in boost/preprocessor/config/config.hpp
+// http://www.boost.org/libs/preprocessor/doc/ref/variadics.html
+// However, supporting compiler detection is not complete. msgpack-c requires
+// variadic macro arguments support. So BOOST_PP_VARIADICS is defined here explicitly.
 #define BOOST_PP_VARIADICS
 #include <boost/preprocessor.hpp>
 
diff --git a/include/msgpack/adaptor/detail/cpp11_define_map.hpp b/include/msgpack/adaptor/detail/cpp11_define_map.hpp
index 58039926..4fd33b68 100644
--- a/include/msgpack/adaptor/detail/cpp11_define_map.hpp
+++ b/include/msgpack/adaptor/detail/cpp11_define_map.hpp
@@ -20,6 +20,10 @@
 
 #if defined(MSGPACK_USE_BOOST)
 
+// BOOST_PP_VARIADICS is defined in boost/preprocessor/config/config.hpp
+// http://www.boost.org/libs/preprocessor/doc/ref/variadics.html
+// However, supporting compiler detection is not complete. msgpack-c requires
+// variadic macro arguments support. So BOOST_PP_VARIADICS is defined here explicitly.
 #define BOOST_PP_VARIADICS
 #include <boost/preprocessor.hpp>