diff --git a/sigc++/signal_base.h b/sigc++/signal_base.h
index 1638642..e43bd5e 100644
--- a/sigc++/signal_base.h
+++ b/sigc++/signal_base.h
@@ -21,6 +21,7 @@
 #ifndef _SIGC_SIGNAL_BASE_H_
 #define _SIGC_SIGNAL_BASE_H_
 
+#include <cstddef>
 #include <list>
 #include <sigc++config.h>
 #include <sigc++/type_traits.h>