Failing with "Parameter 'url' must be a string, not undefined"

Hi,
I am new to Serverless and Node.js. I am trying to run the "aws-node-fetch-file-and-store-in-s3" example. I have not modified the code. Here is how I am invoking it.

serverless invoke -f save -l -d "{'image_url':'https://assets-cdn.github.com/images/modules/open_graph/github-mark.png','key':'github.png'}"

I am getting the following error message.

{
    "errorMessage": "Parameter 'url' must be a string, not undefined",
    "errorType": "TypeError",
    "stackTrace": [
        "Url.parse (url.js:90:11)",
        "urlParse (url.js:84:5)",
        "Body.Request (/var/task/node_modules/node-fetch/lib/request.js:27:16)",
        "/var/task/node_modules/node-fetch/index.js:51:17",
        "new Fetch (/var/task/node_modules/node-fetch/index.js:49:9)",
        "Fetch (/var/task/node_modules/node-fetch/index.js:37:10)",
        "module.exports.save.fetch.then.then.response.then.buffer.then (/var/task/handler.
js:9:3)"
    ]
}

I guess I am doing a silly error, but I am not able to get rid of it. Please help.

Thanks in advance.

Start with the example's handler.js, which appears in the stack trace, and compare the save function's expected event fields with the payload passed to serverless invoke. Reproduce the command against the aws-node-fetch-file-and-store-in-s3 example; done means the URL and key are received and the invocation completes without the undefined-url error.