diff --git a/arch/x86/include/nanok.h b/arch/x86/include/nanok.h
index 8bf0140dcf9..20cdbe2a598 100644
--- a/arch/x86/include/nanok.h
+++ b/arch/x86/include/nanok.h
@@ -744,10 +744,6 @@ typedef struct s_NANO {
 #define STACK_ROUND_UP(x) ROUND_UP(x, STACK_ALIGN_SIZE)
 #define STACK_ROUND_DOWN(x) ROUND_DOWN(x, STACK_ALIGN_SIZE)
 
-#ifndef NULL
-#define NULL (void *)0
-#endif
-
 /* variable declarations */
 
 /*
diff --git a/include/misc/lists.h b/include/misc/lists.h
index bafa56664ed..ae6e5993bdc 100644
--- a/include/misc/lists.h
+++ b/include/misc/lists.h
@@ -54,11 +54,6 @@ struct List {
 };
 
 
-
-#ifndef NULL
-#define NULL (void *)0
-#endif
-
 #ifndef INLINED
 extern void InitList(struct list_head *list);
 extern unsigned int TestListEmpty(struct list_head *list);
diff --git a/include/misc/util.h b/include/misc/util.h
index b791d3bf4ec..4fe1b88ff6f 100644
--- a/include/misc/util.h
+++ b/include/misc/util.h
@@ -58,10 +58,6 @@ extern "C" {
 #define INLINE
 #endif
 
-#ifndef NULL
-#define NULL ((void *)0)
-#endif
-
 #ifndef max
 #define max(a, b) (((a) > (b)) ? (a) : (b))
 #endif
diff --git a/include/nanokernel.h b/include/nanokernel.h
index 763b5a09cb2..4d3aeea5fd9 100644
--- a/include/nanokernel.h
+++ b/include/nanokernel.h
@@ -33,6 +33,7 @@
 #ifndef __NANOKERNEL_H__
 #define __NANOKERNEL_H__
 
+#include <stddef.h>
 #include <stdint.h>
 #include <toolchain.h> /* compiler specific configuration options */
 
diff --git a/kernel/microkernel/include/minik.h b/kernel/microkernel/include/minik.h
index ac2795f5ae4..58a382a002e 100644
--- a/kernel/microkernel/include/minik.h
+++ b/kernel/microkernel/include/minik.h
@@ -33,6 +33,7 @@
 #ifndef MINIK_H
 #define MINIK_H
 
+#include <stddef.h>
 #include <kernel_struct.h>
 #include <kernel_main.h>
 #include <clock_vars.h>