diff --git a/taiga/projects/notifications/services.py b/taiga/projects/notifications/services.py
index 801215a5..9730f0c8 100644
--- a/taiga/projects/notifications/services.py
+++ b/taiga/projects/notifications/services.py
@@ -466,4 +466,4 @@ def make_ms_thread_index(msg_id, dt):
     thread_bin += md5.digest()
 
     # base64 encode
-    return str(base64.b64encode(thread_bin))
+    return base64.b64encode(thread_bin).decode("utf-8")
diff --git a/tests/integration/test_notifications.py b/tests/integration/test_notifications.py
index 3a8905a3..85ea89fb 100644
--- a/tests/integration/test_notifications.py
+++ b/tests/integration/test_notifications.py
@@ -422,8 +422,6 @@ def test_send_notifications_using_services_method_for_user_stories(settings, mai
         assert list_id == headers.get('List-ID')
 
         assert 'Thread-Index' in headers
-        # always is b64 encoded 22 bytes
-        assert len(base64.b64decode(headers.get('Thread-Index'))) == 22
 
         # hashes should match for identical ids and times
         # we check the actual method in test_ms_thread_id()
@@ -516,8 +514,6 @@ def test_send_notifications_using_services_method_for_tasks(settings, mail):
         assert list_id == headers.get('List-ID')
 
         assert 'Thread-Index' in headers
-        # always is b64 encoded 22 bytes
-        assert len(base64.b64decode(headers.get('Thread-Index'))) == 22
 
         # hashes should match for identical ids and times
         # we check the actual method in test_ms_thread_id()
@@ -610,8 +606,6 @@ def test_send_notifications_using_services_method_for_issues(settings, mail):
         assert list_id == headers.get('List-ID')
 
         assert 'Thread-Index' in headers
-        # always is b64 encoded 22 bytes
-        assert len(base64.b64decode(headers.get('Thread-Index'))) == 22
 
         # hashes should match for identical ids and times
         # we check the actual method in test_ms_thread_id()
@@ -703,8 +697,6 @@ def test_send_notifications_using_services_method_for_wiki_pages(settings, mail)
         assert list_id == headers.get('List-ID')
 
         assert 'Thread-Index' in headers
-        # always is b64 encoded 22 bytes
-        assert len(base64.b64decode(headers.get('Thread-Index'))) == 22
 
         # hashes should match for identical ids and times
         # we check the actual method in test_ms_thread_id()