I added a Content-Security-Policy but still the security warning appears

I added a Content-Security-Policy as suggested here: https://www.electronjs.org/docs/tutorial/security#6-define-a-content-security-policy

<html lang="en">
<head>
  <meta http-equiv="Content-Security-Policy" content="default-src 'self'">
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>New Electron App</title>
</head>
<body>
  <span>Our new Electron app</span>
  <div id="root"></div>
</body>
</html>

But still I get this message:
“Electron Security Warning (Insecure Content-Security-Policy). This renderer process has either no Content Security Policy set or a policy with “unsafe-eval” enabled. This exposes users of this app to unnecessary security risks. This warning will not show up once the app is packaged.”

How to solve this security warning?