You can't override specific type of properties in prefab variant?

I have a prefab called Unit, and i made a prefab variant for it called Player. Now inside Unit prefab i have a property called Weapon, a link to a script, like this

Now what i need is to assign an actual value to this property inside Player, using gameobject Weapon, that is only present in Player prefab, and not in Unit prefab. So that inside Unit this property would have the default value None, but Player prefab would have a script assigned to this property.

Here's a screenshot Player prefab variant for clarity:

But i can't do this! It says something like that i can't assign objects from scene to prefab asset. Which is weird, because inside Player i can override any Unit property i want, and keep it as override. Except this kind of properties that refer to gameobjects inside Player but outside Unit.

Why? Is it possible to have a value None inside Unit prefab, but a different value in Player prefab variant? That's how prefab variants are supposed to work, isn't it, they should allow you to override things...