diff --git a/include/boost/program_options/detail/convert.hpp b/include/boost/program_options/detail/convert.hpp
index 0fac187..56b4415 100644
--- a/include/boost/program_options/detail/convert.hpp
+++ b/include/boost/program_options/detail/convert.hpp
@@ -8,6 +8,8 @@
 
 #include <string>
 #include <locale>
+// for mbstate_t
+#include <cwchar>
 
 #include <stdexcept>