Actionable Messages HTTP.Action Returning Error

I have designed a very simple actionable message which looks like this:

{
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "type": "AdaptiveCard",
    "version": "1.0",
    "body": [
        {
            "type": "ActionSet",
            "id": "501324c6-5d4b-3867-ad69-bf0e2b1be718",
            "actions": [
                {
                    "type": "Action.Http",
                    "id": "961548cb-862b-161a-3c46-e1265e740b68",
                    "title": "Approve",
                    "url": "MYURL",
                    "method": "GET"
                }
            ]
        }
    ],
    "padding": "None",
    "@type": "AdaptiveCard",
    "@context": "http://schema.org/extensions"
}

When I click on the button within Outlook, I get the following error:
The remote endpoint returned an error (HTTP 500). Please try again later.

I can see that the URL gets called successfully on my side and calling the same URL in the browser also returns fine with a status 200 so I am not sure why I am getting this error. Any ideas? Do I have to format the response in a certain way from my side so that it knows it was successful?