diff --git a/tiramisu/config.py b/tiramisu/config.py
index 41944f7..91720f7 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -548,11 +548,11 @@ def make_dict(config, flatten=False):
     return options
 
 def mandatory_warnings(config):
-    mandatory = config._cfgimpl_mandatory
-    config._cfgimpl_mandatory = True
+    mandatory = config._cfgimpl_get_toplevel()._cfgimpl_mandatory
+    config._cfgimpl_get_toplevel()._cfgimpl_mandatory = True
     for path in config.getpaths(mandatory=True):
         try:
             value = getattr(config, path)
         except MandatoryError:
             yield path
-    config._cfgimpl_mandatory = mandatory
+    config._cfgimpl_get_toplevel()._cfgimpl_mandatory = mandatory