diff --git a/zerver/lib/test_classes.py b/zerver/lib/test_classes.py
index b0779ed830..4f033fe0a5 100644
--- a/zerver/lib/test_classes.py
+++ b/zerver/lib/test_classes.py
@@ -370,7 +370,7 @@ class ZulipTestCase(TestCase):
         self.assertIn(substring, response.content.decode('utf-8'))
 
     def assert_in_success_response(self, substrings, response):
-        # type: (Iterable[Text], HttpResponse) -> None
+        # type: (List[Text], HttpResponse) -> None
         self.assertEqual(response.status_code, 200)
         decoded = response.content.decode('utf-8')
         for substring in substrings:
diff --git a/zerver/tests/test_email_change.py b/zerver/tests/test_email_change.py
index 4aa6e348b4..d5e67e72be 100644
--- a/zerver/tests/test_email_change.py
+++ b/zerver/tests/test_email_change.py
@@ -31,7 +31,7 @@ class EmailChangeTestCase(ZulipTestCase):
         url = EmailChangeConfirmation.objects.get_activation_url(
             key, 'testserver')
         response = self.client_get(url)
-        self.assert_in_success_response("Whoops", response)
+        self.assert_in_success_response(["Whoops"], response)
 
     def test_confirm_email_change_with_invalid_key(self):
         # type: () -> None
@@ -43,7 +43,7 @@ class EmailChangeTestCase(ZulipTestCase):
         url = EmailChangeConfirmation.objects.get_activation_url(
             key, 'testserver')
         response = self.client_get(url)
-        self.assert_in_success_response("Whoops", response)
+        self.assert_in_success_response(["Whoops"], response)
 
     def test_email_change_when_not_logging_in(self):
         # type: () -> None
@@ -73,7 +73,7 @@ class EmailChangeTestCase(ZulipTestCase):
                                                confirmation_key=key)
         url = EmailChangeConfirmation.objects.get_activation_url(key)
         response = self.client_get(url)
-        self.assert_in_success_response("Whoops", response)
+        self.assert_in_success_response(["Whoops"], response)
 
     def test_confirm_email_change(self):
         # type: () -> None
@@ -93,7 +93,7 @@ class EmailChangeTestCase(ZulipTestCase):
         response = self.client_get(url)
 
         self.assertEqual(response.status_code, 200)
-        self.assert_in_success_response("This confirms that the email address for your Zulip",
+        self.assert_in_success_response(["This confirms that the email address for your Zulip"],
                                         response)
         user_profile = get_user_profile_by_email(new_email)
         self.assertTrue(bool(user_profile))
@@ -115,7 +115,7 @@ class EmailChangeTestCase(ZulipTestCase):
         self.assertEqual(len(mail.outbox), 0)
         result = self.client_post(url, data)
         self.assertEqual(len(mail.outbox), 1)
-        self.assert_in_success_response('We have sent you an email', result)
+        self.assert_in_success_response(['We have sent you an email'], result)
         email_message = mail.outbox[0]
         self.assertEqual(
             email_message.subject,
@@ -127,7 +127,7 @@ class EmailChangeTestCase(ZulipTestCase):
         activation_url = [s for s in body.split('\n') if s][4]
         response = self.client_get(activation_url)
 
-        self.assert_in_success_response("This confirms that the email address",
+        self.assert_in_success_response(["This confirms that the email address"],
                                         response)
 
     def test_unauthorized_email_change(self):
@@ -153,7 +153,7 @@ class EmailChangeTestCase(ZulipTestCase):
         self.assertEqual(len(mail.outbox), 0)
         result = self.client_post(url, data)
         self.assertEqual(len(mail.outbox), 1)
-        self.assert_in_success_response('We have sent you an email', result)
+        self.assert_in_success_response(['We have sent you an email'], result)
         email_message = mail.outbox[0]
         self.assertEqual(
             email_message.subject,
diff --git a/zerver/tests/test_signup.py b/zerver/tests/test_signup.py
index a54d65f57c..7e09fcb1fb 100644
--- a/zerver/tests/test_signup.py
+++ b/zerver/tests/test_signup.py
@@ -972,7 +972,7 @@ class UserSignUpTest(ZulipTestCase):
 
         # Pick a password and agree to the ToS.
         result = self.submit_reg_form_for_user(email, password, full_name="<invalid>")
-        self.assert_in_success_response("Invalid characters in name!", result)
+        self.assert_in_success_response(["Invalid characters in name!"], result)
 
     def test_unique_completely_open_domain(self):
         # type: () -> None