Does anyone know how to successfully create an (initial) synced block?

I have tried the following:

 string pageId="...";
 IList<ISyncedBlockChildrenRequest> blocks=...
 var content = new SyncedBlockBlockRequest()
 {
 SyncedBlock = new()
 {
 Children = blocks,
 SyncedFrom = null
 }
 };
 var result=await notionClient.Blocks.AppendChildrenAsync(new BlockAppendChildrenRequest(){BlockId = pageId, Children = [content] });

The SyncedFrom=null should not be necessary as it is the default value.

This fails with the following exception:

Notion.Client.NotionApiException: body failed validation: body.children[0].synced_block.synced_from should be defined, instead was undefined.

I guess the problem is that the null value for SyncedFrom is serialized as a missing property when sent to the Notion API, thereby being undefined rather than a Javascript null, but I don't know how to send an actual null. Or is there something else I am doing wrong?