diff --git a/rest_framework/mixins.py b/rest_framework/mixins.py
index 1edcfa5c9..8dc0c3295 100644
--- a/rest_framework/mixins.py
+++ b/rest_framework/mixins.py
@@ -113,6 +113,10 @@ class UpdateModelMixin(object):
             slug_field = self.get_slug_field()
             setattr(obj, slug_field, slug)
 
+        # Ensure we clean the attributes so that we don't eg return integer
+        # pk using a string representation, as provided by the url conf kwarg.
+        obj.full_clean()
+
 
 class DestroyModelMixin(object):
     """
diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 02377492c..8156bc185 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -497,25 +497,27 @@ class ModelSerializer(Serializer):
         if instance is not None:
             for key, val in attrs.items():
                 setattr(instance, key, val)
-            return instance
 
-        # Reverse relations
-        for (obj, model) in self.opts.model._meta.get_all_related_m2m_objects_with_model():
-            field_name = obj.field.related_query_name()
-            if field_name in attrs:
-                self.m2m_data[field_name] = attrs.pop(field_name)
+        else:
+            # Reverse relations
+            for (obj, model) in self.opts.model._meta.get_all_related_m2m_objects_with_model():
+                field_name = obj.field.related_query_name()
+                if field_name in attrs:
+                    self.m2m_data[field_name] = attrs.pop(field_name)
 
-        # Forward relations
-        for field in self.opts.model._meta.many_to_many:
-            if field.name in attrs:
-                self.m2m_data[field.name] = attrs.pop(field.name)
+            # Forward relations
+            for field in self.opts.model._meta.many_to_many:
+                if field.name in attrs:
+                    self.m2m_data[field.name] = attrs.pop(field.name)
+
+            instance = self.opts.model(**attrs)
 
-        instance = self.opts.model(**attrs)
         try:
             instance.full_clean(exclude=self.get_validation_exclusions())
         except ValidationError, err:
             self._errors = err.message_dict
             return None
+
         return instance
 
     def save(self, save_m2m=True):
diff --git a/rest_framework/tests/generics.py b/rest_framework/tests/generics.py
index 14fa66f93..7c24d84ea 100644
--- a/rest_framework/tests/generics.py
+++ b/rest_framework/tests/generics.py
@@ -175,7 +175,7 @@ class TestInstanceView(TestCase):
         content = {'text': 'foobar'}
         request = factory.put('/1', json.dumps(content),
                               content_type='application/json')
-        response = self.view(request, pk=1).render()
+        response = self.view(request, pk='1').render()
         self.assertEquals(response.status_code, status.HTTP_200_OK)
         self.assertEquals(response.data, {'id': 1, 'text': 'foobar'})
         updated = self.objects.get(id=1)