average_score field is not a `SerializerMethodField`

from django.db.models import Avg

from rest_framework import serializers

from . import models


class GameSerializer(serializers.ModelSerializer):

    class Meta:
        average_score = serializers.SerializerMethodField

        fields = ('id', 'name', 'avarage_score')
        model = models.Game

    def get_average_score(self, obj):
        average = obj.Game.scores.all.aggregate(Avg('score')).get('score__avg')
        if average is None:
            return 0
        return round(average*2) / 2

class PlayerSerializer(serializers.ModelSerializer):
    class Meta:
        fields = ('id', 'name', 'age', 'parent_approval')
        model = models.Player

    def validate(self, data):
        if data['age'] < 13 and not data['parent_approval']:
            raise serializers.ValidationError(
                "Children under 13 require parental approval"
            )
        return data


class ScoreSerializer(serializers.ModelSerializer):
    game = serializers.HyperlinkedRelatedField(
        read_only=True, view_name='apiv2:game-detail')
    player = serializers.HyperlinkedRelatedField(
        read_only=True, view_name='apiv2:player-detail')

    class Meta:
        fields = ('id', 'game', 'player', 'score')
        model = models.Score