# File lib/test/unit/assertions.rb, line 1769
          def prepare_for_diff(from, to)
            if !from.is_a?(String) or !to.is_a?(String)
              from = convert(from)
              to = convert(to)
            end

            if diff_target_string?(from) and diff_target_string?(to)
              from = ensure_diffable_string(from)
              to = ensure_diffable_string(to)
              [from, to]
            else
              [nil, nil]
            end
          end