insertRecords method: mandatory fields (Contacts)

Hello,

I'm inserting Contacts to Zoho CRM.
I found information that for Contacts mandatory field is Last Name (https://www.zoho.com/crm/help/api/modules-fields.html#Contacts). But I noticed that I can put Contact without Last Name, if it has Email, but I can't insert it if it doesn't have Email. Parameter duplicateCheck is set to 2, newFormat to 1 (I tried also with 2) and version to 2/4.

Is this my incomprehension or Zoho documentation is wrong?

Thanks to look at this,
Eve

EDIT:
If I try to put to Zoho CRM Contact without mail, in response is no error. It looks as follows:

<?xml version="1.0" encoding="UTF-8"?>
<response uri="/crm/private/xml/Contacts/insertRecords">
  <result/>
</response>