diff --git a/src/sim/mathexpr.hh b/src/sim/mathexpr.hh
index d857fa5124..6d04d25388 100644
--- a/src/sim/mathexpr.hh
+++ b/src/sim/mathexpr.hh
@@ -41,6 +41,7 @@
 #define __SIM_MATHEXPR_HH__
 
 #include <algorithm>
+#include <functional>
 #include <string>
 
 class MathExpr {