diff --git a/src/kernel/diff_cnstrs.cpp b/src/kernel/diff_cnstrs.cpp
index ebab6ef697..6e7c96dc54 100644
--- a/src/kernel/diff_cnstrs.cpp
+++ b/src/kernel/diff_cnstrs.cpp
@@ -8,6 +8,7 @@ Author: Leonardo de Moura
 #include <utility>
 #include <unordered_map>
 #include <vector>
+#include <functional>
 #include "util/hash.h"
 #include "util/safe_arith.h"
 #include "kernel/diff_cnstrs.h"