diff --git a/include/tscore/Ptr.h b/include/tscore/Ptr.h
index be50b1404ec..0f221c831cb 100644
--- a/include/tscore/Ptr.h
+++ b/include/tscore/Ptr.h
@@ -25,6 +25,8 @@
 
 #include "tscore/ink_atomic.h"
 
+#include <cstddef>
+
 ////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////
 //////              ATOMIC VERSIONS