--- a/include/bitcoin/bitcoin/wallet/dictionary.hpp
+++ b/include/bitcoin/bitcoin/wallet/dictionary.hpp
@@ -22,6 +22,7 @@
 #include <array>
 #include <vector>
 #include <bitcoin/bitcoin/compat.hpp>
+#include <cstddef>
 
 namespace libbitcoin {
 namespace wallet {
