diff --git a/src/kernel/expr.h b/src/kernel/expr.h
index 4cc583985..b420df9dd 100644
--- a/src/kernel/expr.h
+++ b/src/kernel/expr.h
@@ -7,6 +7,7 @@ Author: Leonardo de Moura
 #pragma once
 #include <iostream>
 #include <limits>
+#include <atomic>
 #include "rc.h"
 #include "name.h"
 #include "level.h"