Formula field: referencing to parent module's fields

Formula field: referencing to parent module's fields

Please advice on the following scenario:

There are two custom modules created: Device, Subscription.
Device-Subscription is child-parent relationship (subscription can have many devices):
device has 'subscription' lookup field.

Now for device module I need to add field that refers to subscription.'start date' field (just for convinience)
In order to do this for device module I  create field of formula type and set the following expression: ${Devices.Subscription.Start Date}. but the validation is getting failed: Given field label does not exist
(see attachment  #1)

But the field 'Start Date' does exist in Subscription module:
(see attachment #2)

Is is possible to use nested(parent) module's fields in formulas?
What am I doing wrong?

Thanks.
Anton