diff --git a/cosmic_ray/counting.py b/cosmic_ray/counting.py
index <HASH>..<HASH> 100644
--- a/cosmic_ray/counting.py
+++ b/cosmic_ray/counting.py
@@ -15,6 +15,7 @@ class _CountingCore:
 
     def visit_mutation_site(self, node, op):
         self.count += 1
+        return node
 
     def repr_args(self):
         return []
CountingCore.visit_mutation_site now returns node.

Prior to this it wasn't returning the node it was passed, and this meant
it was short-circuiting mutation by trimming of most of the AST!