diff --git a/loopy/kernel/instruction.py b/loopy/kernel/instruction.py
index 73760978c21ee92625bb396edeb078c2be790d4d..61127232a9f494fe2fdc536dd50d8fdf41b8f17c 100644
--- a/loopy/kernel/instruction.py
+++ b/loopy/kernel/instruction.py
@@ -1326,6 +1326,9 @@ class CInstruction(InstructionBase):
                 for a in self.assignees)
 
     def with_transformed_expressions(self, f, assignee_f=None):
+        if assignee_f is None:
+            assignee_f = f
+
         return self.copy(
                 iname_exprs=[
                     (name, f(expr))