diff --git a/src/library/normalize.cpp b/src/library/normalize.cpp
index 0aea314ad7..1486f140b1 100644
--- a/src/library/normalize.cpp
+++ b/src/library/normalize.cpp
@@ -4,6 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 
 Author: Leonardo de Moura
 */
+#include <string>
 #include "util/interrupt.h"
 #include "util/name_generator.h"
 #include "kernel/type_checker.h"