DS-160: No Surname in passport

While filling DS160 online form i was giving my spouse information wherein i was asked to provide her surname and given name. The details should be strictly as per passport. There is no surname in my spouse passport. It is blank and there is only given name. But the surname field in DS160 online form is mandatory. When surname is optional in indian passport i wonder why it is mandatory in DS160 form. Please let me how should i proceed filling DS160 form.

In this case,

first name = FNU

last name = entire name