diff --git a/django/forms/models.py b/django/forms/models.py
index 13aea4acab..869138c922 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -292,13 +292,16 @@ class BaseModelForm(BaseForm):
             elif field in self._errors.keys():
                 exclude.append(f.name)
 
-            # Exclude empty fields that are not required by the form. The
-            # underlying model field may be required, so this keeps the model
-            # field from raising that error.
+            # Exclude empty fields that are not required by the form, if the
+            # underlying model field is required. This keeps the model field
+            # from raising a required error. Note: don't exclude the field from
+            # validaton if the model field allows blanks. If it does, the blank
+            # value may be included in a unique check, so cannot be excluded
+            # from validation.
             else:
                 form_field = self.fields[field]
                 field_value = self.cleaned_data.get(field, None)
-                if field_value is None and not form_field.required:
+                if not f.blank and not form_field.required and field_value in EMPTY_VALUES:
                     exclude.append(f.name)
         return exclude
 
diff --git a/tests/modeltests/model_forms/mforms.py b/tests/modeltests/model_forms/mforms.py
index 5777b4cb6e..aef763ef62 100755
--- a/tests/modeltests/model_forms/mforms.py
+++ b/tests/modeltests/model_forms/mforms.py
@@ -1,6 +1,7 @@
+from django import forms
 from django.forms import ModelForm
 
-from models import Product, Price, Book, DerivedBook, ExplicitPK, Post, DerivedPost
+from models import Product, Price, Book, DerivedBook, ExplicitPK, Post, DerivedPost, Writer
 
 class ProductForm(ModelForm):
     class Meta:
@@ -30,3 +31,9 @@ class PostForm(ModelForm):
 class DerivedPostForm(ModelForm):
     class Meta:
         model = DerivedPost
+
+class CustomWriterForm(ModelForm):
+   name = forms.CharField(required=False)
+
+   class Meta:
+       model = Writer
diff --git a/tests/modeltests/model_forms/tests.py b/tests/modeltests/model_forms/tests.py
index fb4e8adb45..6a5f9395cc 100644
--- a/tests/modeltests/model_forms/tests.py
+++ b/tests/modeltests/model_forms/tests.py
@@ -2,7 +2,8 @@ import datetime
 from django.test import TestCase
 from django import forms
 from models import Category, Writer, Book, DerivedBook, Post
-from mforms import ProductForm, PriceForm, BookForm, DerivedBookForm, ExplicitPKForm, PostForm, DerivedPostForm
+from mforms import (ProductForm, PriceForm, BookForm, DerivedBookForm, 
+                   ExplicitPKForm, PostForm, DerivedPostForm, CustomWriterForm)
 
 
 class IncompleteCategoryFormWithFields(forms.ModelForm):
@@ -37,6 +38,10 @@ class ValidationTest(TestCase):
         form = IncompleteCategoryFormWithExclude(data={'name': 'some name', 'slug': 'some-slug'})
         assert form.is_valid()
 
+    def test_notrequired_overrides_notblank(self):
+        form = CustomWriterForm({})
+        assert form.is_valid()
+
 # unique/unique_together validation
 class UniqueTest(TestCase):
     def setUp(self):