DRF Serializer fields DictField() AttributeError:

In django rest framework If i use DictField() in serializer its given me AttributeError at /api/v1/analytics/reports-configuration/1/ ‘str’ object has no attribute ‘items’ error.please take a look.

serializers.py

import json
from rest_framework import serializers

from models import ReportConfiguration


class ReportConfigurationSerializer(serializers.ModelSerializer):
    report_type = serializers.ChoiceField(choices=['query', 'activity', 'summary'])
    read_only = serializers.BooleanField(required=True)
    query_data = serializers.DictField(required=True)

    class Meta:
        model = ReportConfiguration

        fields = ('id', 'title', 'query_data', 'created_by', 'last_activity_by', 'deleted',
                  'deleted_by', 'added_at', 'last_activity_at', 'report_type', 'read_only')


    def validate(self, data):
        if data['report_type'] is not 'summary' and data['read_only'] is True:
            raise serializers.ValidationError("If read only true then report type should be summary")
        if data['report_type'] is 'summary' and data['read_only'] is not True:
            raise serializers.ValidationError("If read only not true then report type should not be summary")
        return data

    def create(self, validated_data):
        validated_data['created_by'] = self.context['request'].user
        validated_data['last_activity_by'] = self.context['request'].user
        return ReportConfiguration.objects.create(**validated_data)

views.py

class ReportConfigurationViewSet(viewsets.ModelViewSet):
    serializer_class = ReportConfigurationSerializer
    queryset = ReportConfiguration.objects.all()
    permission_classes = (permissions.IsAuthenticated,)
    filter_backends = (DjangoFilterBackend,)
    filter_class = ReportFilter

    def list(self, request):
        response = super(ReportConfigurationViewSet, self).list(request)
        for data in response.data['results']:
            data['query_data'] = json.loads(data['query_data'])
        return response

    def retrieve(self, request, pk=None):
        response = super(ReportConfigurationViewSet, self).retrieve(request, pk=pk)
        response.data['query_data'] = json.loads(response.data['query_data'])
        return response

models.py

class ReportConfiguration(models.Model):
    class ReportTypeChoice(Choice):
        QueryReport = 'query'
        ActivityReport = 'activity'
        SummaryList = 'summary'

    title = models.CharField(max_length=300, unique=True, blank=False, null=False)
    query_data = JSONField(default={})
    created_by = models.ForeignKey(User, null=True, blank=True, related_name='report_created_by')
    last_activity_by = models.ForeignKey(User, null=True, blank=True,related_name='last_active_in_report')
    deleted = models.BooleanField(default=False)
    deleted_by = models.ForeignKey(User, null=True, blank=True, related_name='report_deleted_by')
    added_at = models.DateTimeField(auto_now_add=True)
    last_activity_at = models.DateTimeField(default=timezone.now)
    report_type = models.CharField(max_length=15, choices=ReportTypeChoice)
    read_only = models.BooleanField(default=False)

Error:

Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
ERROR Internal Server Error: /api/v1/analytics/reports-configuration/1/
Traceback (most recent call last):
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/viewsets.py", line 83, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/views.py", line 477, in dispatch
    response = self.handle_exception(exc)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/views.py", line 437, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/views.py", line 474, in dispatch
    response = handler(request, *args, **kwargs)
  File "/Users/ranvijay.s/wk/project/django/dhanalytics/views.py", line 47, in retrieve
    response = super(ReportConfigurationViewSet, self).retrieve(request, pk=pk)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/mixins.py", line 58, in retrieve
    return Response(serializer.data)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/serializers.py", line 527, in data
    ret = super(Serializer, self).data
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/serializers.py", line 262, in data
    self._data = self.to_representation(self.instance)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/serializers.py", line 496, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/Users/ranvijay.s/.virtualenvs/my_project/lib/python2.7/site-packages/rest_framework/fields.py", line 1601, in to_representation
    for key, val in value.items()
AttributeError: 'str' object has no attribute 'items'