diff --git a/xs/xsp/Print.xsp b/xs/xsp/Print.xsp
index 478b1faf8..d9872aa7e 100644
--- a/xs/xsp/Print.xsp
+++ b/xs/xsp/Print.xsp
@@ -142,14 +142,13 @@ _constant()
         %code%{ 
             // Touching every config as the Perl bindings does not correctly export ModelConfig,
             // therefore the configs have often invalid timestamps.
-            model->config.touch();
             for (auto obj : model->objects) {
                 obj->config.touch();
                 for (auto vol : obj->volumes)
                     vol->config.touch();
             }
             for (auto mat : model->materials)
-                mat->config.touch();
+                mat.second->config.touch();
             RETVAL = THIS->apply(*model, *config);
         %};
     bool has_infinite_skirt();