diff --git a/src/branch_tier.hpp b/src/branch_tier.hpp
index ba88a061..352584d9 100644
--- a/src/branch_tier.hpp
+++ b/src/branch_tier.hpp
@@ -19,8 +19,8 @@ public:
   std::vector<Branch2>::iterator begin() { return _branches.begin(); }
   std::vector<Branch2>::iterator end() { return _branches.end(); }
 
-  std::vector<Branch2>::iterator begin() const { return _branches.begin(); }
-  std::vector<Branch2>::iterator end() const { return _branches.end(); }
+  std::vector<Branch2>::const_iterator begin() const { return _branches.begin(); }
+  std::vector<Branch2>::const_iterator end() const { return _branches.end(); }
   
 
 private: