diff --git a/test/tribool_io_test.cpp b/test/tribool_io_test.cpp
index d7254f1..84bce90 100644
--- a/test/tribool_io_test.cpp
+++ b/test/tribool_io_test.cpp
@@ -8,6 +8,7 @@
 #include <sstream>
 #include <string>
 #include <iostream>
+#include <ios>     // for std::boolalpha
 
 #ifndef BOOST_NO_STD_LOCALE
 #  include <locale>