UnityWebRequest Certificate Handler causes "Unknown Error"

I want to validate the certificate of the response of web requests I make from UnityWebRequest. Without certificateHandler line it gets job done and works just right. When I add the line 8 it returns network error, which is called "Unknown Error". Is this a bug, or am I missing something?

Code (CSharp):

1.  
2. public class WebRequest
3. {
4.   using (UnityWebRequest request = new UnityWebRequest("https://www.url.com/list", "POST"))
5.   {
6.     request.uploadHandler = new UploadHandlerRaw(someData);
7.     request.downloadHandler = new DownloadHandlerBuffer();
8.  
9.     //problem line
10.     request.certificateHandler = new RequestCertificate();
11.  
12.     yield return request.SendWebRequest();
13.  
14.     Debug.Log(request.error);
15.   }
16. }
17. ...
18.  
19. internal class RequestCertificate : CertificateHandler
20. {
21.   protected override bool ValidateCertificate(byte[] certificateData)
22.   {
23.     return true;
24.   }
25. }
26.  

 

Yes using this exact code I run - except for the url and someData var. Code does not even enter the breakpoint at return true line in the Validatecert method.

2018.3.14f1, in Editor, Windows 10