diff --git a/aes.h b/aes.h
index ade29e8..b29b668 100644
--- a/aes.h
+++ b/aes.h
@@ -2,6 +2,7 @@
 #define _AES_H_
 
 #include <stdint.h>
+#include <stddef.h>
 
 // #define the macros below to 1/0 to enable/disable the mode of operation.
 //