Setting date field to null if string date value is empty or null

Hi,

I'm recollecting data from CRM and updating in Creator. Some fields could be empty - no problem with strings. But date fields, when they are null, I haven't found the correct way for setting them. I've tried like this:

r = zoho.crm.searchRecords("Contacts", "(Last Name|=|" + last_name + ")");
contact = r.get(0);
new_id = insert into Persona
[
      Added_User = zoho.loginuser
      ... (more fields)
      Fecha_Alta_Persona = if (c.get("Fecha Alta") != null, c.get("Fecha Alta").toDate(), null)
      ... (more fields)
];

But, it doesn't compile: "Both the expression should have same datatype".

Is it not possible to set null to date field?

Thank you very much,

Bernat