VAF1b, but blank lastname on passport

[needTier1]

Hi Everyone,

I am looking to seek your advice on Family visitor visa situation for my Parents.

My father passport's lastname field is blank, he never actually used last name(bank statement and property documents has same format),
and I do have last name of my passport. My Mother passport has her lastname before marriage.

Hence, I just want to check if there would be any issue if I keep lastname as blank on my father's vaf1b form?

or is it best idea to just spent some time and get both passport corrected. (I mean correct lastname similar to mine on both passports).

Just to note, they do have travelled abraod (Middle east) before.

any help would be appreciated.

Thanks