diff --git a/tdigest/include/tdigest.hpp b/tdigest/include/tdigest.hpp
index 38f3b1a5..bb4f7376 100644
--- a/tdigest/include/tdigest.hpp
+++ b/tdigest/include/tdigest.hpp
@@ -115,7 +115,7 @@ class tdigest {
    * Merge the given t-Digest into this one
    * @param other t-Digest to merge
    */
-  void merge(tdigest& other);
+  void merge(const tdigest& other);
 
   /**
    * Process buffered values and merge centroids if needed
diff --git a/tdigest/include/tdigest_impl.hpp b/tdigest/include/tdigest_impl.hpp
index afc64ef3..165bda6c 100644
--- a/tdigest/include/tdigest_impl.hpp
+++ b/tdigest/include/tdigest_impl.hpp
@@ -44,7 +44,7 @@ void tdigest<T, A>::update(T value) {
 }
 
 template<typename T, typename A>
-void tdigest<T, A>::merge(tdigest& other) {
+void tdigest<T, A>::merge(const tdigest& other) {
   if (other.is_empty()) return;
   vector_centroid tmp(buffer_.get_allocator());
   tmp.reserve(buffer_.size() + centroids_.size() + other.buffer_.size() + other.centroids_.size());