diff --git a/Utils/IOutputStream.hh b/Utils/IOutputStream.hh
index 524a8eba66fa9cc164898e3aa4a9746ce8e4d373..4fe89e6bdbbbac9fbbad578d55effb0570b05900 100644
--- a/Utils/IOutputStream.hh
+++ b/Utils/IOutputStream.hh
@@ -5,6 +5,7 @@
 
 #include <string>
 #include <vector>
+#include <cstddef>
 #include <stdint.h>
 #include <cinttypes>
 #include <limits.h>