diff --git a/test/optional_test_emplace.cpp b/test/optional_test_emplace.cpp
index d44e1d1..5182d97 100644
--- a/test/optional_test_emplace.cpp
+++ b/test/optional_test_emplace.cpp
@@ -15,6 +15,7 @@
 #pragma hdrstop
 #endif
 
+#include <string>
 #include "boost/core/lightweight_test.hpp"
 #include "boost/none.hpp"
 
diff --git a/test/optional_test_minimum_requirements.cpp b/test/optional_test_minimum_requirements.cpp
index 1e22735..11ff247 100644
--- a/test/optional_test_minimum_requirements.cpp
+++ b/test/optional_test_minimum_requirements.cpp
@@ -15,6 +15,7 @@
 #pragma hdrstop
 #endif
 
+#include <string>
 #include "boost/core/lightweight_test.hpp"
 #include "boost/none.hpp"