diff --git a/lib/system/nuttx/io.c b/lib/system/nuttx/io.c
index ab9bc6ab..c21d7c82 100644
--- a/lib/system/nuttx/io.c
+++ b/lib/system/nuttx/io.c
@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
+#include <stddef.h>
 #include <metal/cache.h>
 #include <metal/io.h>
 #include <nuttx/arch.h>