diff --git a/libsrc/cbm/penadjust.c b/libsrc/cbm/penadjust.c
index 104c33c8d..f33b3a275 100755
--- a/libsrc/cbm/penadjust.c
+++ b/libsrc/cbm/penadjust.c
@@ -5,6 +5,7 @@
 */
 
 
+#include <stddef.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <mouse.h>