diff --git a/src/branch_tier.hpp b/src/branch_tier.hpp
index bbf680bd..4fe7d8b8 100644
--- a/src/branch_tier.hpp
+++ b/src/branch_tier.hpp
@@ -10,6 +10,7 @@ class BranchTier
 {
 public:
   BranchTier();
+  BranchTier(BranchTier &);
   BranchTier(toml::value const &);
 
 public: