diff --git a/tests/include/unittest.hh b/tests/include/unittest.hh
index 7c65f9f..3ce4f38 100644
--- a/tests/include/unittest.hh
+++ b/tests/include/unittest.hh
@@ -16,7 +16,7 @@
 #include <iostream>
 #include <iomanip>
 #include <cstdlib>
-
+#include <string>
 
 std::ostream & print_hex(
     std::ostream & os,