gistfile1.diff

diff --git a/tests/model_forms/tests.py b/tests/model_forms/tests.py
index 09c62c5..d64295c 100644
--- a/tests/model_forms/tests.py
+++ b/tests/model_forms/tests.py
@@ -1800,3 +1800,24 @@ class M2mHelpTextTest(TestCase):
         html = form.as_p()
         self.assertInHTML('<ul id="id_status">', html)
         self.assertInHTML(dreaded_help_text, html, count=0)
+
+
+from django.forms import ModelForm
+from django.db import models
+class Person(models.Model):
+    age = models.IntegerField(
+        error_messages={'invalid': 'Custom message.'},
+    )
+
+class ContactForm(ModelForm):
+    class Meta:
+        model = Person
+        fields = ('age',)
+
+class ContactTest(TestCase):
+    def test_contact(self):
+        form = ContactForm({'age': 'abc'})
+        self.assertHTMLEqual(
+            str(form.errors['age']),
+            '<ul class="errorlist"><li>Custom message.</li></ul>',
+        )

gistfile2.pytb

$ PYTHONPATH=. python3 ./tests/runtests.py --settings=test_sqlite model_forms
Creating test database for alias 'default'...
Creating test database for alias 'other'...
F.............................s........................
======================================================================
FAIL: test_contact (model_forms.tests.ContactTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/loic/Dev/django/tests/model_forms/tests.py", line 1822, in test_contact
    '<ul class="errorlist"><li>Custom message.</li></ul>',
  File "/Users/loic/Dev/django/django/test/testcases.py", line 635, in assertHTMLEqual
    self.fail(self._formatMessage(msg, standardMsg))
AssertionError: <ul class="errorlist">
<li>
Enter a whole number.
</li>
</ul> != <ul class="errorlist">
<li>
Custom message.
</li>
</ul>
  <ul class="errorlist">
  <li>
- Enter a whole number.
+ Custom message.
  </li>
  </ul>

----------------------------------------------------------------------
Ran 55 tests in 0.310s

FAILED (failures=1, skipped=1)
Destroying test database for alias 'default'...
Destroying test database for alias 'other'...