declared(params) - "no implicit conversion of Symbol into Integer" when receiving blank array

1. I have an optional array parameter. In some cases I want it to be blank, but if the items are passed, they must match the schema inside.

params do
  optional :outline, type: Array do
    requires :x
    requires :y
  end
end

2. I do a PATCH request and send {"outline": []} payload.
3. Received params =>{ "outline"=>[""] } - here is the first question - why an empty string inside an array? I have a feeling that I've struggled with that a few years back, but don't remember what was the cause.
4. declared(params) raises no implicit conversion of Symbol into Integer

Start by reproducing the PATCH request with the declared optional array schema and an empty outline payload, then trace the declared(params) handling that produces the empty string and Symbol-to-Integer error. Done means the empty array is handled without the exception and the expected behavior for items inside the array is covered by a regression test.