diff --git a/src/library/definitional/equations.cpp b/src/library/definitional/equations.cpp
index 1b8175cf7..67cf95af2 100644
--- a/src/library/definitional/equations.cpp
+++ b/src/library/definitional/equations.cpp
@@ -4,6 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 
 Author: Leonardo de Moura
 */
+#include <string>
 #include "kernel/expr.h"
 #include "library/kernel_serializer.h"