Return the category value as null in Django (json)

Question

Why does my category return null when I receive Jason output? After serializing and receiving the code as Jason, I receive the category name null. An example of the codes is shown below.

**my serializer**

from .models import Post, Category
from rest_framework import serializers
from django.contrib.auth import get_user_model


class CategorySerializer(serializers.ModelSerializer):
class Meta:
    model = Category
    fields = ["id", "name", "image"]


class AuthorSerializer(serializers.ModelSerializer):
class Meta:
    model = get_user_model()
    fields = ["id", "username", "email"]


class PostSerializer(serializers.ModelSerializer):
category = serializers.CharField(source="category.name", 
read_only=True)
author = serializers.CharField(source="author.username", 
read_only=True)

class Meta:
    model = Post
    fields = "__all__"


class UserSerializer(serializers.ModelSerializer):
class Meta:
    model = get_user_model()
    fields = "__all__"