custom_fields: Value must be a JSON object and cannot be null

When I tried the update task for non-premium users I got this error.
“custom_fields: Value must be a JSON object and cannot be null”

Why can I set start_at and start_on on null but not custom_fields?

Used endpoint

PUT https://app.asana.com/api/1.0/tasks/

{
   "data":{
      "due_at":"2023-12-06T01:00:00+0000",
      "due_on":null,
      "start_at":null,
      "start_on":null,
      "custom_fields":null,
      "completed":true,
      "name":"add test",
      "notes":"as",
      "assignee":null
   }
}

Also, I tried to set "custom_fields":{} but then I got
“custom Fields are not available for free users or guests”

If you’re able to set start_at and start_on even just to null in a Free account, I actually think that’s a bug; Free accounts do not have access to those properties.

For sure you won’t be able to set custom fields in a Free account, since again those aren’t available to Free accounts.

You’re not able to set start_at or start_on to an actual non-null date/time, are you?

When I try to set start_on I get error
“premium_only_setting_task_start_date” - “Setting start_on is not available”

A fun fact is that if due_on or due_at is empty the error is
“You must provide due_on or due_at when setting start_on.” not premium_only_setting_task_start_date