diff --git a/py65/testio.c02 b/py65/testio.c02
index 9acb532..27fa540 100644
--- a/py65/testio.c02
+++ b/py65/testio.c02
@@ -3,6 +3,7 @@
  **********************************************/
  
 #include <py65.h02>
+#include <stddef.h02>
 #include <stdio.h02>
 
 char key;      //Key read from keyboard
diff --git a/py65/teststr.c02 b/py65/teststr.c02
index 2853ea1..9399ed8 100644
--- a/py65/teststr.c02
+++ b/py65/teststr.c02
@@ -3,6 +3,7 @@
  ******************************************/
  
 #include <py65.h02>
+#include <stddef.h02>
 #include <stdio.h02>
 #include <string.h02>