diff --git a/tools/cc.py b/tools/cc.py
index 6a17b16..6236b11 100644
--- a/tools/cc.py
+++ b/tools/cc.py
@@ -219,7 +219,9 @@ class RegBank:
             self.loaded(var1, reg0)
 
     def snap(self):
-        return dict(self.vars), dict(self.cleanup)
+        cleanup = collections.defaultdict(list)
+        cleanup.update({r: list(c) for r, c in self.cleanup.items()})
+        return dict(self.vars), cleanup
 
     def restore(self, snap):
         self.reset()