diff --git a/include/libtorrent/ed25519.hpp b/include/libtorrent/ed25519.hpp
index 917b84a9d..f8a66a695 100644
--- a/include/libtorrent/ed25519.hpp
+++ b/include/libtorrent/ed25519.hpp
@@ -2,7 +2,7 @@
 #define ED25519_HPP
 
 #include "libtorrent/aux_/export.hpp" // for TORRENT_EXPORT
-#include <stddef.h> // for size_t
+#include <cstddef> // for ptrdiff_t, size_t
 
 namespace libtorrent {