diff --git a/src/strass/strass_app/utils.py b/src/strass/strass_app/utils.py
index 7268d2c641263fe0830783a2732d5e4dddfbd5c1..694ffd9674829c05c19e9c3fe9520e402d8444ae 100644
--- a/src/strass/strass_app/utils.py
+++ b/src/strass/strass_app/utils.py
@@ -271,7 +271,7 @@ def validate_multiple_email(email):
 
 
 def _pdf_object_cleanup(pdf_file, obj):
-    for k in list(obj):
+    for k in list(obj.keys()):
         if k in [
             '/JavaScript',
             '/S',