Unresolved reference when importing from sibling sub-package with

I have following similar directory structure.

main.py
pack
   |___sub_pack1
|__a.py
   |___sub_pack2
|__b.py

Now inside main.py (which is my main program that I execute) I am importing like follow
from pack.sub_pack1 import a

Working fine.

Inside a.py I am importing like follow
from pack.sub_pack2 import b

At this point pycharm underlines above code as red and notifying me "Unresolved reference pack".

Now my code is working fine which should. I am curious why pycharm considering it as error and what can I do to avoid such thing.

Thanks
ansumanbebarta