build model on association creates unexpected results

Say we have this model

class Car < ActiveRecord::Base
  # string  name
  # integer wheels
end

Executing following code

Car.where(:name  => "Jeff").new #=> <Car name: "Jeff", wheels: nil>
Car.where(:wheels => [4,8]).new #=> <Car name: nil, wheels: 1>

The first case yield expected result, however the second one does not! 1 wheel?

Car.where(:wheels => 4..8).new #=> <Car name: nil, wheels: nil>

When using a range we don't get the issue.

Test in patch

My suggestion would be to be more careful what values get passed to the scope_for_create (relation.rb:359) method. How this should be achieved, that I do not know.