inheritance tax form IHT436 unused residence nil rate band

this form asks for "Inheritance Tax reference number for spouse or civil partner’s estate" but the spouse in my case left a small estate (less than 100k) which did not incur any inheritance tax and therefore no IHT400 form was completed and no IHT number was ever applied for or given. So how do we fill in this box?

Hi,

Please contact the Inheritance Tax team for advice.

Inheritance Tax: general enquiries

Thank you.