Problem with case statement and mb_chars in Ruby-1.9.2

RoR-3.0.1

I have this case statement:

  case n.hll_normalize
    when "common name"
      @my_correspondent.correspondent_common_name = s
    when "legal name"
      . . .
    else
      raise ArgumentError, "#{n} attribute is not provided for."
  end

hll_normalize is defined as:

  def hll_normalise
    strip.squeeze(" ").mb_chars.downcase
  end

I pass this case statement this value for n: "common name"

When run with Ruby-1.8.7p302 this works. The exact same code run with
Ruby-1.9.2p0 yields:

common name attribute is not provided for. (ArgumentError)

When I enter rails console then I see this:

$ rails c
Loading development environment (Rails 3.0.1)
ruby-1.9.2-p0 > x = "common name"
=> "common name"
ruby-1.9.2-p0 > x.mb_chars
=> common name
ruby-1.9.2-p0 > x.mb_chars.downcase
=> common name
ruby-1.9.2-p0 > "common name" == x.mb_chars.downcase
=> true
ruby-1.9.2-p0 > "common name" == x.strip.squeeze(" ").mb_chars.downcase
=> true
ruby-1.9.2-p0 > case x.strip.squeeze(" ").mb_chars.downcase
ruby-1.9.2-p0 ?> when "uncommon" then puts "nothing"
ruby-1.9.2-p0 ?> when "common name" then puts x
ruby-1.9.2-p0 ?> end
=> nil
ruby-1.9.2-p0 >

So, what is the case statement sensitive wrt mb_chars that changes the
result of "common name" == x.strip.squeeze(" ").mb_chars.downcase when
used as an argument? Is there a nil coming back from somewhere in the
case statement argument evaluation when mb_chars is involved?

This is definitely related to mb_chars because this works:

ruby-1.9.2-p0 > case x.strip.squeeze(" ").downcase
ruby-1.9.2-p0 ?> when "uncommon" then puts "nothing"
ruby-1.9.2-p0 ?> when "common name" then puts x
ruby-1.9.2-p0 ?> end
common name
=> nil
ruby-1.9.2-p0 >