User Flow - API connector cannot parse response

I have an Azure B2C user flow. It is associated with an API Connector pointing to an Azure Function. The function returns a ResponseContent with extension claims:

public class ResponseContent
{
    public const string ApiVersion = "1.0.0";

    public ResponseContent()
    {
        this.version = ResponseContent.ApiVersion;
        this.action = "Continue";
    }

    public ResponseContent(string action, string userMessage)
    {
        this.version = ResponseContent.ApiVersion;
        this.action = action;
        this.userMessage = userMessage;
    }

    public ResponseContent(string userTypes, string accountIdentifiers, string pricebookAuthorized, string portalAuthorized)
    {
        this.version = ResponseContent.ApiVersion;
        this.action = "Continue";
        this.extension_UserTypes = userTypes;
        this.extension_AccountIdentifiers = accountIdentifiers;
        this.extension_PricebookAuthorized = pricebookAuthorized;
        this.extension_PortalAuthorized = portalAuthorized;
    }

    public string version { get; }
    public string action { get; set; }

    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string userMessage { get; set; }

    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string extension_UserTypes { get; set; }

    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string extension_AccountIdentifiers { get; set; }

    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string extension_PricebookAuthorized { get; set; }

    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public string extension_PortalAuthorized { get; set; }
}

Here are the claims of the user flow:

When I run this Azure function using Postman, the following is returned:

{
  "version": "1.0.0",
  "action": "Continue",
  "extension_UserTypes": "",
  "extension_AccountIdentifiers": "",
  "extension_PricebookAuthorized": "",
  "extension_PortalAuthorized": ""
}

But when I try to run the user flow on Azure, I get

Microsoft.Identity.Client.MsalServiceException: AADB2C90261: The claims exchange 'PreSendClaimsRestful' specified in step '2' returned HTTP error response that could not be parsed.

What might be wrong, and how this can be diagnosed?