Surname left blank in the passport

[doubleA]

For some reason, my mother's passport has the surname section left blank, and instead has her entire name (that is first name and last name) under the section "Other names".

Now, since the DS-156 says Surname (as in passport), does it have to be left blank too??? Else should she go ahead include her last name under the surname section although the passport does not have it that way?

Will appreciate any thoughts in this regard. Thanks!

[dahlia]

put first name unknown (FNU) in DS 156 and put the other name as surname.

[doubleA]

put first name unknown (FNU) in DS 156 and put the other name as surname.

Dahlia, thanks for your reply.
But wouldnt putting the first name on paspsort as last name on the application be a contradiction?? Please let me know.