我在dynamodb中将对象推入数组/将对象设置为docs状态时遇到问题。

Invalid UpdateExpression: Incorrect number of operands for operator or function; operator or function: if_not_exists, number of operands: 1

我做错什么了？我很感激你的帮助！

export async function postTrade(position) {
    let params = {
        Key: {
            PK: 'stocks' 
        },
        UpdateExpression: 'set #tickers = list_append(if_not_exists(#tickers))',
        ExpressionAttributeNames: {
            '#tickers': 'tickers'
        },
        ExpressionAttributeValues: {
            ':tickers': [position]
        },
        TableName: 'stockTable'
    };
    await docClient.update(params).promise();
}

迪纳摩唱片

{
 "PK": "stocks",
 "tickers": [
 ]
}