diff --git a/python_modules/codegen.py b/python_modules/codegen.py
index 02ffdb9..c470988 100644
--- a/python_modules/codegen.py
+++ b/python_modules/codegen.py
@@ -357,6 +357,7 @@ class CodeWriter:
         return index
 
     def push_index(self):
+        assert self.current_index > 0
         self.current_index = self.current_index - 1
 
     class Index: