diff --git a/pymbolic/mapper/c_code.py b/pymbolic/mapper/c_code.py
index 6790b4f2038e3787050806cd7f62dd42d3cd3bad..dc36040568276c18ed009df86b4073f2da97b66f 100644
--- a/pymbolic/mapper/c_code.py
+++ b/pymbolic/mapper/c_code.py
@@ -64,7 +64,7 @@ class CCodeMapper(SimplifyingSortingStringifyMapper):
 
     def map_common_subexpression(self, expr, enclosing_prec):
         try:
-            cse_name = self.cse_to_name[expr]
+            cse_name = self.cse_to_name[expr.child]
         except KeyError:
             from pymbolic.mapper.stringifier import PREC_NONE
             cse_str = self.rec(expr.child, PREC_NONE)
@@ -91,4 +91,6 @@ class CCodeMapper(SimplifyingSortingStringifyMapper):
             self.cse_to_name[expr.child] = cse_name
             self.cse_names.add(cse_name)
 
+            assert len(self.cse_names) == len(self.cse_to_name)
+
         return cse_name