Custom deserializer does not output Nullable types

[woppa684]

According to this page, nullable types should be possible:

• T is a class or a struct.
• All public fields in T are either
  • One of [sbyte, byte, short, ushort, int, uint, long, DateTime, string, float, double] or their nullable equivalents.
  • Another struct or class following the same rules.
  • Array of type T2 that follows the same rules.
  • IListT2 where T2 follows the same rules.
  • Doesn't have any recursive types.

When trying to output a class like

public class Test
{
    public int? OptionalInt { get; set; }
}

I get the following errors:

{
    "eventName": "streamingNode0.output-output#0$0",
    "time": "2023-06-05T09:12:59.1792641Z",
    "resourceId": "",
    "operationName": "Send Events",
    "category": "Execution",
    "status": "Failed",
    "level": "Warning",
    "properties": {
        "Message": "Output adapter has encountered an error while preparing data to be sent.",
        "Type": "OutputAdapterError",
        "Correlation ID": "de3007a5-072b-42b1-b4b4-6f5000b05a56",
        "Error": "- Cannot convert value of type 'System.Int32' to JSON!\r\n"
    }
}

{
    "eventName": "output-output#0$0",
    "time": "2023-06-05T09:12:59.1638651Z",
    "resourceId": "output",
    "operationName": "",
    "category": "UnsupportedDataTypeError",
    "status": "",
    "level": "Warning",
    "properties": {
        "BriefMessage": "Column [OptionalInt] contains value with unsupported data type 'System.Int32'",
        "Message": "Column [OptionalInt] contains value with unsupported data type 'System.Int32'",
        "EventCount": 0
    }
}

Changing int? to int makes it work as expected. Am I doing something wrong or is this not supported?