diff --git a/Octree/include/CGAL/Octree.h b/Octree/include/CGAL/Octree.h
index 3b06c07979c..457c1657706 100644
--- a/Octree/include/CGAL/Octree.h
+++ b/Octree/include/CGAL/Octree.h
@@ -401,7 +401,7 @@ public:
    * \param rhs tree to compare with
    * \return whether the trees have the same topology
    */
-  bool operator==(Octree<PointRange, PointMap> &rhs) {
+  bool operator==(Octree<PointRange, PointMap> &rhs) const {
 
     // Identical trees should have the same bounding box
     if (rhs.m_bbox_min != m_bbox_min || rhs.m_bbox_side != m_bbox_side)