diff --git a/nano/lib/blocks.hpp b/nano/lib/blocks.hpp
index 0c780c9a4..d16d0ca40 100644
--- a/nano/lib/blocks.hpp
+++ b/nano/lib/blocks.hpp
@@ -9,6 +9,8 @@
 
 #include <boost/property_tree/ptree_fwd.hpp>
 
+#include <optional>
+
 typedef struct blake2b_state__ blake2b_state;
 
 namespace nano