diff --git a/simgear/emesary/ITransmitter.hxx b/simgear/emesary/ITransmitter.hxx
index 0b4c6386..b3418534 100644
--- a/simgear/emesary/ITransmitter.hxx
+++ b/simgear/emesary/ITransmitter.hxx
@@ -20,6 +20,7 @@
 *
 *---------------------------------------------------------------------------*/
 
+#include <cstddef>
 #include <simgear/structure/SGSharedPtr.hxx>
 namespace simgear
 {