Invalid Response Received Invalid JSON Response Received

My addon activates just fine, and even gets the event details for the callback; however, regardless of what I return (nothing, a full HTML page, or pieces of HTML) I get the following error in the modal window/iframe:

Invalid Response Received

Invalid JSON Response Received

The iframe's src is https://addon.go.servicem8.com/PluginPlatformExternalSDK_HandleActionEvent?&s_strAddonId=######&s_strActionType=job&s_strEventName=######&s_form_values=jobUUID-companyUUID&s_auth=######&jobUUID=######&companyUUID=######