Empty Cloud Functions params

Question

I'm calling an IBM Cloud Functions web action

function main(params) {
    params = JSON.stringify(params)
    return { message: 'params: ' + params };
}

using this POST request

https://us-south.functions.cloud.ibm.com/api/v1/namespaces/<space>/actions/order?order=123&blocking=true

but I don't get the params:

"response": {
    "result": {
        "message": "params: {}"
    },
    "status": "success",
    "success": true
},