[Rails] when does 'dog' not equal 'dog'?

This is more a Ruby than a Rails question -- feel free to redirect me
as appropriate -- but either this is a Ruby 1.9 bug or I have some
serious misunderstanding. Essentially, I've caught the <=> operator
returning nil -- not -1, 0, or 1, but nil. Schematically:

class A
attr_accessor :slot
fun matches?(s)
p "s <=> @slot = #{s <=> @slot}"
return (s <=> @slot) == 0
end
end

Driven from a script, I get something like this:

# Case 1: Notice that <=> evaluates to nil when it should eval to 0
a.slot = "dog"
a.matches?("dog")
"s <=> @slot = "
=> false

# Case 2: When I pass a String.new("dog") everything works
a.slot = "dog"
a.matches?(String.new("dog"))
"s <=> @slot = 0"
=> true

What's really odd is that called interactively, Case 1 works as
expected. When called from a script, it exhibits the oddity above.

Am I missing something fundamental about Ruby and string comparison?

- ff

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