diff --git a/gtsam.h b/gtsam.h
index e679b6838..e500c6328 100644
--- a/gtsam.h
+++ b/gtsam.h
@@ -164,7 +164,7 @@ class KeySet {
 
   // structure specific methods
   void insert(size_t key);
-  void merge(gtsam::KeySet& other);
+  void merge(const gtsam::KeySet& other);
   bool erase(size_t key); // returns true if value was removed
   bool count(size_t key) const; // returns true if value exists