diff --git a/include/boost/system/detail/error_code.ipp b/include/boost/system/detail/error_code.ipp
index ec9fbb4..803ba7b 100644
--- a/include/boost/system/detail/error_code.ipp
+++ b/include/boost/system/detail/error_code.ipp
@@ -384,7 +384,7 @@ namespace
             boost::detail::winapi::MAKELANGID_(boost::detail::winapi::LANG_NEUTRAL_,
             boost::detail::winapi::SUBLANG_DEFAULT_), // Default language
             &buf[0],
-            buf.size(),
+            static_cast<DWORD>(buf.size()),
             NULL
         );