6 files changed, 53 insertions, 4 deletions
diff --git a/.travis.yml b/.travis.yml
index 18fe66ab..2e6ed46a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,7 +7,7 @@ python:
   - "3.3"
 
 env:
-  - DJANGO="django==1.6"
+  - DJANGO="django==1.6.2"
   - DJANGO="django==1.5.5"
   - DJANGO="django==1.4.10"
   - DJANGO="django==1.3.7"
diff --git a/rest_framework/relations.py b/rest_framework/relations.py
index 02185c2f..163a8984 100644
--- a/rest_framework/relations.py
+++ b/rest_framework/relations.py
@@ -33,6 +33,7 @@ class RelatedField(WritableField):
     many_widget = widgets.SelectMultiple
     form_field_class = forms.ChoiceField
     many_form_field_class = forms.MultipleChoiceField
+    null_values = (None, '', 'None')
 
     cache_choices = False
     empty_label = None
@@ -168,9 +169,9 @@ class RelatedField(WritableField):
                 return
             value = [] if self.many else None
 
-        if value in (None, '') and self.required:
-            raise ValidationError(self.error_messages['required'])
-        elif value in (None, ''):
+        if value in self.null_values:
+            if self.required:
+                raise ValidationError(self.error_messages['required'])
             into[(self.source or field_name)] = None
         elif self.many:
             into[(self.source or field_name)] = [self.from_native(item) for item in value]
diff --git a/rest_framework/tests/serializers.py b/rest_framework/tests/serializers.py
new file mode 100644
index 00000000..cc943c7d
--- /dev/null
+++ b/rest_framework/tests/serializers.py
@@ -0,0 +1,8 @@
+from rest_framework import serializers
+
+from rest_framework.tests.models import NullableForeignKeySource
+
+
+class NullableFKSourceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = NullableForeignKeySource
diff --git a/rest_framework/tests/test_nullable_fields.py b/rest_framework/tests/test_nullable_fields.py
new file mode 100644
index 00000000..6ee55c00
--- /dev/null
+++ b/rest_framework/tests/test_nullable_fields.py
@@ -0,0 +1,30 @@
+from django.core.urlresolvers import reverse
+
+from rest_framework.compat import patterns, url
+from rest_framework.test import APITestCase
+from rest_framework.tests.models import NullableForeignKeySource
+from rest_framework.tests.serializers import NullableFKSourceSerializer
+from rest_framework.tests.views import NullableFKSourceDetail
+
+
+urlpatterns = patterns(
+    '',
+    url(r'^objects/(?P<pk>\d+)/$', NullableFKSourceDetail.as_view(), name='object-detail'),
+)
+
+
+class NullableForeignKeyTests(APITestCase):
+    """
+    DRF should be able to handle nullable foreign keys when a test
+    Client POST/PUT request is made with its own serialized object.
+    """
+    urls = 'rest_framework.tests.test_nullable_fields'
+
+    def test_updating_object_with_null_fk(self):
+        obj = NullableForeignKeySource(name='example', target=None)
+        obj.save()
+        serialized_data = NullableFKSourceSerializer(obj).data
+
+        response = self.client.put(reverse('object-detail', args=[obj.pk]), serialized_data)
+
+        self.assertEqual(response.data, serialized_data)
diff --git a/rest_framework/tests/views.py b/rest_framework/tests/views.py
new file mode 100644
index 00000000..3917b74a
--- /dev/null
+++ b/rest_framework/tests/views.py
@@ -0,0 +1,8 @@
+from rest_framework import generics
+from rest_framework.tests.models import NullableForeignKeySource
+from rest_framework.tests.serializers import NullableFKSourceSerializer
+
+
+class NullableFKSourceDetail(generics.RetrieveUpdateDestroyAPIView):
+    model = NullableForeignKeySource
+    model_serializer_class = NullableFKSourceSerializer
diff --git a/rest_framework/throttling.py b/rest_framework/throttling.py
index a946d837..efa9fb94 100644
--- a/rest_framework/throttling.py
+++ b/rest_framework/throttling.py
@@ -136,6 +136,8 @@ class SimpleRateThrottle(BaseThrottle):
             remaining_duration = self.duration
 
         available_requests = self.num_requests - len(self.history) + 1
+        if available_requests <= 0:
+            return None
 
         return remaining_duration / float(available_requests)