Assignments to field of type 'Lookup'

In my main table I have a field A that is a lookup to LookupTableA. That LookupTableA has a field AB that is a lookup to LookupTableB.

The main table also has a field B that is a lookup to LookupTableB.

When importing new data only the value for field A is provided and I want to fill field B with an assignment that uses the information present in LookupTableA.

What I tried to do is the following assignment:

IF(IS_NULL(B.\[Record\]), A.AB.\[Record\])

This assignment checks whether or not B is already filled and if not tries to fill it with the lookup record of the linked LookupTableA entry.

Sadly that assignment returns an error. Is there something wrong in my assignment?

It works if I use a fixed entry from LookupTableB like this:

IF(IS_NULL(B.\[Record\]), B \[xyz\])

It seems strange that the first version does not work, although it also points to a specific entry in LookupTableB.

Would be glad if anybody has some thoughts on how to achieve that more dynamic assignment described above.