djangorestframework serializer errors: {u'non_field_errors': [u'No input provided']}

I'm using djangorestframework, and someone makes a PUT request to a .../peoplelist/2/markAsSeen, passing in just a Person object's id in the URL. I fetch the Person object (2 in this case) and then simply change the fetched Person object's field has_been_viewed to True. The updated Person object is then to be serialized and returned back to the client.

if request.method == 'PUT':
    serializer = PersonSerializer(person,partial=True)#person is a valid object here
    if serializer.is_valid():
        serializer.save()
        return Response(serializer.data)
    else:
        return Response(serializer.errors,)

serializer errors is {u'non_field_errors': [u'No input provided']} serializer.data looks fine to me

class PersonSerializer(serializers.ModelSerializer):

    class Meta:
        model = Person
        fields = ('id',)