diff --git a/src/branch_tier.hpp b/src/branch_tier.hpp
index c5300973..0d586c3d 100644
--- a/src/branch_tier.hpp
+++ b/src/branch_tier.hpp
@@ -13,7 +13,7 @@ public:
   BranchTier(toml::value const &);
 
 public:
-  void copy_enabled_rw(BranchTier &);
+  void copy_enabled_rw(BranchTier &) const;
 
 public:
   uint64_t min_free_space;