diff --git a/test/test_to_string.cpp b/test/test_to_string.cpp
index 1870697e..844c9790 100644
--- a/test/test_to_string.cpp
+++ b/test/test_to_string.cpp
@@ -28,6 +28,7 @@ SOFTWARE.
 
 #include "unit_test_framework.h"
 
+#include <sstream>
 #include <iomanip>
 #include <limits>
 #include <ostream>