Django serializers ManyToManyField Error

Django Backend
I can't change my user model with api. I get the following 400 error:
{'friends': [ErrorDetail(string='Incorrect type. Expected pk value, received str.', code='incorrect_type')]}

models.py
class User(AbstractUser):
friends = models.ManyToManyField('User',blank=True)

serializers.py
class UserSerializer(serializers.ModelSerializer):
class Meta:
model = User
fields=('id','friends','username','full_name','first_name','last_name')

class UserById(APIView):
def put(self, request, pk=None):
user = User.objects.get(pk=pk)
serializer = UserSerializer(user, data=request.data)
if serializer.is_valid():
serializer.save()
info_message={'msgs':{'info':f'updated'}}
return Response(data=info_message, status=200)
return Response(serializer.errors, status=400)