diff --git a/src/branches2.hpp b/src/branches2.hpp
index a9c8ff55..93af2d6f 100644
--- a/src/branches2.hpp
+++ b/src/branches2.hpp
@@ -4,6 +4,7 @@
 #include "branch_tier.hpp"
 
 #include "toml.hpp"
+#include "nonstd/optional.hpp"
 
 #include <vector>