要正确使用aiohttp.post()方法中的json参数，请使用dict类型而不是字符串类型。例如，以下代码会引发上述错误：

async def post_json_data():
    url = 'http://example.com/api'
    data = '{"name": "John", "age": 30}'
    async with aiohttp.ClientSession() as session:
        async with session.post(url, json=data) as response:
            return await response.json()

要解决这个问题，需要将data变量从字符串类型转换为dict类型。例如，以下代码将不会引发上述错误：

async def post_json_data():
    url = 'http://example.com/api'
    data = {"name": "John", "age": 30}
    async with aiohttp.ClientSession() as session:
        async with session.post(url, json=data) as response:
            return await response.json()

如果你必须使用字符串类型的json数据，那么可以使用json.loads()方法将其转换为dict类型，例如：

import json

async def post_json_data():
    url = 'http://example.com/api'
    data = '{"name": "John", "age": 30}'
    async with aiohttp.ClientSession() as session:
        async with session.post(url, json=json.loads(data)) as response:
            return await response.json()