diff --git a/app/celery/nightly_tasks.py b/app/celery/nightly_tasks.py
index 30df04de8..97c1fa59a 100644
--- a/app/celery/nightly_tasks.py
+++ b/app/celery/nightly_tasks.py
@@ -295,14 +295,14 @@ def letter_raise_alert_if_no_ack_file_for_zip():
                                               subfolder=today_str + '/zips_sent',
                                               suffix='.TXT'):
         subname = key.split('/')[-1]  # strip subfolder in name
-        zip_file_set.add(subname.upper().rstrip('ZIP.TXT'))
+        zip_file_set.add(subname.upper().replace('.ZIP.TXT', ''))
 
     # get acknowledgement file
     ack_file_set = set()
 
     for key in s3.get_list_of_files_by_suffix(bucket_name=current_app.config['DVLA_RESPONSE_BUCKET_NAME'],
                                               subfolder='root/dispatch', suffix='.ACK.txt', last_modified=yesterday):
-        ack_file_set.add(key.lstrip('root/dispatch').upper().rstrip('ACK.TXT'))
+        ack_file_set.add(key.lstrip('root/dispatch').upper().replace('.ACK.TXT', ''))
 
     message = (
         "Letter ack file does not contain all zip files sent. "
diff --git a/tests/app/celery/test_nightly_tasks.py b/tests/app/celery/test_nightly_tasks.py
index 7f07592f5..f0c8b8c4b 100644
--- a/tests/app/celery/test_nightly_tasks.py
+++ b/tests/app/celery/test_nightly_tasks.py
@@ -57,10 +57,10 @@ def mock_s3_get_list_match(bucket_name, subfolder='', suffix='', last_modified=N
 
 def mock_s3_get_list_diff(bucket_name, subfolder='', suffix='', last_modified=None):
     if subfolder == '2018-01-11/zips_sent':
-        return ['NOTIFY.2018-01-11175007.ZIP.TXT', 'NOTIFY.2018-01-11175008.ZIP.TXT', 'NOTIFY.2018-01-11175009.ZIP.TXT',
-                'NOTIFY.2018-01-11175010.ZIP.TXT']
+        return ['NOTIFY.2018-01-11175007p.ZIP.TXT', 'NOTIFY.2018-01-11175008.ZIP.TXT',
+                'NOTIFY.2018-01-11175009.ZIP.TXT', 'NOTIFY.2018-01-11175010.ZIP.TXT']
     if subfolder == 'root/dispatch':
-        return ['root/disoatch/NOTIFY.2018-01-11175007.ACK.TXT', 'root/disoatch/NOTIFY.2018-01-11175008.ACK.TXT']
+        return ['root/disoatch/NOTIFY.2018-01-11175007p.ACK.TXT', 'root/disoatch/NOTIFY.2018-01-11175008.ACK.TXT']
 
 
 @freeze_time('2016-10-18T10:00:00')