diff --git a/crypto/ed25519-donna/ed25519.h b/crypto/ed25519-donna/ed25519.h
index b5fe78950..46544dda0 100644
--- a/crypto/ed25519-donna/ed25519.h
+++ b/crypto/ed25519-donna/ed25519.h
@@ -1,6 +1,8 @@
 #ifndef ED25519_H
 #define ED25519_H
 
+#include <stddef.h>
+
 #include "options.h"
 
 #if defined(__cplusplus)