`object_id` Cannot Be Used in `let`

The following simple spec does not compile for me:

require "spectator"

Spectator.describe String do
  describe "this is a test" do
    let(object_id) { "123" }
  end
end

Changing object_id to anything else (e.g. object_idx) works.

The ultimate error is:

In /usr/local/Cellar/crystal/0.35.1/src/reference.cr:73:15

 73 | object_id.to_s(io, 16)
                ^---
Error: wrong number of arguments for 'String#to_s' (given 2, expected 0..1)

Overloads are:
 - String#to_s(io : IO)
 - String#to_s()
 - Reference#to_s(io : IO)
 - Object#to_s(io : IO)
 - Object#to_s()