diff --git a/Classification/include/CGAL/Classification/Sum_of_weighted_features_classifier.h b/Classification/include/CGAL/Classification/Sum_of_weighted_features_classifier.h
index f9ea789d8b6..904072790ac 100644
--- a/Classification/include/CGAL/Classification/Sum_of_weighted_features_classifier.h
+++ b/Classification/include/CGAL/Classification/Sum_of_weighted_features_classifier.h
@@ -684,7 +684,7 @@ public:
     }
 
     // Write property tree to XML file
-    boost::property_tree::xml_writer_settings<std::string> settings(' ', 3);
+    boost::property_tree::xml_writer_settings<boost::property_tree::ptree::key_type> settings(' ', 3);
     boost::property_tree::write_xml(output, tree, settings);
   }