diff --git a/nmigen/back/pysim.py b/nmigen/back/pysim.py
index fb234b5..47d84fa 100644
--- a/nmigen/back/pysim.py
+++ b/nmigen/back/pysim.py
@@ -295,7 +295,7 @@ class Simulator:
             self._domain_signals[domain] = ValueSet()
 
         hierarchy = {}
-        def add_fragment(fragment, scope=("top",)):
+        def add_fragment(fragment, scope=()):
             hierarchy[fragment] = scope
             for subfragment, name in fragment.subfragments:
                 add_fragment(subfragment, (*scope, name))