type of class instance variable

[znz]

rbs prototype rb always generates untyped for class instance variables.
And rbs subtract does not remove the type of class instance variables.

% rbs --version
rbs 3.8.1
% cat class_instance_variable.rb
class C
  @civ = []
end
% cat sig/class_instance_variable.rbs
class C
  self.@civ: Array
end
% rbs prototype rb class_instance_variable.rb -o prototype
Processing `class_instance_variable.rb`...
  Generating RBS for `class_instance_variable.rb`...
    - Writing RBS to `prototype/class_instance_variable.rbs`...
% cat prototype/class_instance_variable.rbs
class C
  self.@civ: untyped
end
% rbs subtract --write prototype sig
% cat prototype/class_instance_variable.rbs
class C
  self.@civ: untyped
end

How can I empty prototype after rbs subtract with class instance variables?