diff --git a/lib/libmio0/mio0.h b/lib/libmio0/mio0.h
index e2692fc..715bf7f 100644
--- a/lib/libmio0/mio0.h
+++ b/lib/libmio0/mio0.h
@@ -1,4 +1,5 @@
 #pragma once
+#include <stddef.h>
 
 // defines