safe_constantize raise exception instead of return nil

Question

def xxx(class)
  cl = "#{class}".safe_constantize
  return '' unless cl
  cl.name
end

my spec

context 'class exist' do
  it 'return instance of ' do
    xxx("String").should be_eql "String"
  end
end

context 'class does not exist' do
  it 'return empty string' do
    xxx("foo").should be_eql ''
  end
end

Then when run my test first(class exist) pass but second fail surprisingly with exeption

Failure/Error: xxx
  NameError:
    uninitialized constant foo

thats is wierd because documentation says:

# +safe_constantize+ tries to find a declared constant with the name specified
# in the string. It returns nil when the name is not in CamelCase
# or is not initialized.  See ActiveSupport::Inflector.safe_constantize
#
#   'Module'.safe_constantize  # => Module
#   'Class'.safe_constantize   # => Class
#   'blargle'.safe_constantize # => nil

activesupport (4.1.7)

rspec (2.13.0)

rails (4.1.7)

ruby (2.1.2)

I suppose save_constantize is raising exection insted of return nil.