Node SDK 400 response - The 'recipient' parameter is required

try {
    const { requestId } = await courier.send({
      message: {
        to: {
          slack: {
            access_token: accessToken,
            channel: channel,
          },
        },
        template: template,
        data,
      },
    })

    return { slackNotificationRequestId: requestId }
  } catch (e) {
    console.dir({ e }, { depth: 3 })
  }

This throws an 400 error -The ‘recipient’ parameter is required.