TLS peer's certificate is not trusted; reason = unable to get local issuer certificate

I’m fetching https://novascotia.ca/coronavirus/data/COVID-19-data.csv from my worker but the response text is always

<!DOCTYPE html>

<html>
  <head>
    <title>Error communicating with origin</title>
    <link rel="stylesheet" id="cf_styles-css" href="https://cloudflareworkers.com/cdn-cgi/styles/cf.errors.css" type="text/css" media="screen,projection" />
    <style type="text/css">body{margin:0;padding:0}</style>
  </head>
  <body>
    <div id="cf-wrapper">
      <div id="cf-error-details" class="cf-error-details-wrapper">
        <div class="cf-wrapper cf-header cf-error-overview">
          <h1>
            <span class="cf-error-type" data-translate="error">Uh oh...</span>
          </h1>
          <h2 class="cf-subheadline" data-translate="error_desc">Error communicating with origin server novascotia.ca</h2>
        </div>

        <section></section><!-- spacer -->

        <div class="cf-section cf-wrapper">
          <h2 data-translate="what_happened">Details</h2>
          <p><code style="font-family:monospace">TLS peer's certificate is not trusted; reason = unable to get local issuer certificate</code></p>
        </div>

        <div class="cf-error-footer cf-wrapper">
          <p>
            <span class="cf-footer-item"><span data-translate="performance_security_by">Performance &amp; security by</span> <a href="https://www.cloudflare.com" target="_blank">Cloudflare</a></span>
          </p>
        </div><!-- /.error-footer -->
      </div>
    </div>

  </body>
</html>

I checked the website’s SSL certificate and it looks ok from Firefox and Chrome.

  const response = await fetch('https://novascotia.ca/coronavirus/data/COVID-19-data.csv')
  const csv = await response.text()
  console.log(csv)