foo.diff

diff --git a/django/core/exceptions.py b/django/core/exceptions.py
index cf4aea0..2f02c14 100644
--- a/django/core/exceptions.py
+++ b/django/core/exceptions.py
@@ -141,9 +141,9 @@ class ValidationError(Exception):
                 for field, errors in self.error_dict.items():
                     error_dict.setdefault(field, []).extend(errors)
             else:
-                error_dict = self.error_dict
+                error_dict.update(self.error_dict)
         else:
-            error_dict[NON_FIELD_ERRORS] = self.error_list
+            error_dict.setdefault(NON_FIELD_ERRORS, []).extend(self.error_list)
         return error_dict
 
     def __iter__(self):
diff --git a/tests/forms_tests/tests/test_forms.py b/tests/forms_tests/tests/test_forms.py
index 985ff8e..6782c80 100644
--- a/tests/forms_tests/tests/test_forms.py
+++ b/tests/forms_tests/tests/test_forms.py
@@ -740,6 +740,24 @@ class FormsTestCase(TestCase):
         with six.assertRaisesRegex(self, ValueError, "has no field named"):
             f.add_error('missing_field', 'Some error.')
 
+    def test_backwards_compat_update_error_dict(self):
+        class CodeForm(Form):
+            code = CharField(max_length=10)
+
+            def clean(self):
+                try:
+                    raise ValidationError({
+                        'code': [ValidationError('Code error')]
+                    })
+                except ValidationError as e:
+                    self._errors = e.update_error_dict(self._errors)
+                    for field, error_list in self._errors.items():
+                        if not isinstance(error_list, self.error_class):
+                            self._errors[field] = self.error_class(error_list)
+
+        form = CodeForm({'code': 'hello'})
+        self.assertEqual(dict(form.errors), {'code': ['Code error']})
+
     def test_has_error(self):
         class UserRegistration(Form):
             username = CharField(max_length=10)