When I try to map a nested property from source (long) to the target (Long) and the source or the nested object is null, the target gets the value 0L instead of null. I think the type of generated method in the impl is wrong. Instead of using the type of the target (Long) which actually is nullable, MapStruct uses the type of the source property (long) for the generated method.

I attached a simple model and a unit-test which covers this issue:
src.zip