diff --git a/src/cls/rbd/cls_rbd_client.cc b/src/cls/rbd/cls_rbd_client.cc
index 87b5c28e1fb..9ad132bd1ce 100644
--- a/src/cls/rbd/cls_rbd_client.cc
+++ b/src/cls/rbd/cls_rbd_client.cc
@@ -2120,7 +2120,7 @@ namespace librbd {
         return r;
       }
 
-      bufferlist::iterator iter = out_bl.begin();
+      auto iter = out_bl.cbegin();
       return mirror_image_map_list_finish(&iter, image_mapping);
     }