diff --git a/html/tree.h b/html/tree.h
index d0d49b4..7654e85 100644
--- a/html/tree.h
+++ b/html/tree.h
@@ -46,6 +46,7 @@
 #define tree_hh_

 #include <cassert>
+#include <cstddef>
 #include <memory>
 #include <stdexcept>
 #include <iterator>