diff --git a/src/cpp_regex_traits.cpp b/src/cpp_regex_traits.cpp
index 5601cf40..24b9e10e 100644
--- a/src/cpp_regex_traits.cpp
+++ b/src/cpp_regex_traits.cpp
@@ -37,6 +37,7 @@
 #include <cstdio>
 #include <list>
 #include <cctype>
+#include <ios>
 #include <iostream>
 #include <map>
 #include <boost/regex/regex_traits.hpp>