a[0] is a t020 and also not null.
a[1] is a t020 and also a k and also not null.
a[2] is a t020 and also null.
a[3] is a t020 and also null.

Assigning a[0] to u failed.
Assigning a[1] to u worked ok.
Assigning a[2] to u worked ok.
Assigning a[3] to u worked ok.