diff --git a/wagtail/wagtailimages/fields.py b/wagtail/wagtailimages/fields.py
index 36351caf7f..93c0ac950f 100644
--- a/wagtail/wagtailimages/fields.py
+++ b/wagtail/wagtailimages/fields.py
@@ -46,10 +46,8 @@ class WagtailImageField(ImageField):
         )
 
         self.error_messages['file_too_large'] = _(
-            "This file is too big (%%(max_upload_size)s). Maximum filesize %s."
-        ) % {
-            'max_upload_size': max_upload_size_text,
-        }
+            "This file is too big (%%s). Maximum filesize %s."
+        ) % max_upload_size_text
 
     def check_image_file_format(self, f):
         # Check file extension
diff --git a/wagtail/wagtailimages/tests/test_admin_views.py b/wagtail/wagtailimages/tests/test_admin_views.py
index 940d5b544a..fa0748d9ed 100644
--- a/wagtail/wagtailimages/tests/test_admin_views.py
+++ b/wagtail/wagtailimages/tests/test_admin_views.py
@@ -1,5 +1,4 @@
 import json
-import unittest
 
 from django.test import TestCase, override_settings
 from django.utils.http import urlquote
@@ -96,7 +95,6 @@ class TestImageAddView(TestCase, WagtailTestUtils):
         # The form should have an error
         self.assertFormError(response, 'form', 'file', "This field is required.")
 
-    @unittest.expectedFailure
     @override_settings(WAGTAILIMAGES_MAX_UPLOAD_SIZE=1)
     def test_add_too_large_file(self):
         response = self.post({