Error sending headers webhook

When I send the attribute headers, I’m getting error 500

mutation {
    createWebhook(input:{
        actions: ["card.done"], 
        name: "SAP", 
        url: "http://x.x.x.x:8080/v1/public/sap-integrator/invoice",
        headers: {Authorization:"Basic xxx"},
        pipe_id: 302174055
    }){
        webhook {actions, headers, id, name, url}
    }
}