diff --git a/folly/Math.h b/folly/Math.h
index c7dd7fe1e..1c5406836 100644
--- a/folly/Math.h
+++ b/folly/Math.h
@@ -21,6 +21,8 @@
 
 #pragma once
 
+#include <cstddef>
+
 #include <stdint.h>
 
 #include <cmath>