rails.lighthouseapp.com/projects/8994/tickets/6225-memcachestore-cant-deal-with-umlauts-and-special-characters The error is caused by charcter encodings that can't be compared with ASCII-8BIT regular expressions and by special characters like the umlaut in UTF-8.

Methods
T
Instance Public methods
test_common_utf8_values()

Source: show | on GitHub 

# File activesupport/test/caching_test.rb, line 422
def test_common_utf8_values
  key = "\xC3\xBCmlaut".force_encoding(Encoding::UTF_8)
  assert @cache.write(key, "1", :raw => true)
  assert_equal "1", @cache.read(key)
  assert_equal "1", @cache.fetch(key)
  assert @cache.delete(key)
  assert_equal "2", @cache.fetch(key, :raw => true) { "2" }
  assert_equal 3, @cache.increment(key)
  assert_equal 2, @cache.decrement(key)
end
test_retains_encoding()

Source: show | on GitHub 

# File activesupport/test/caching_test.rb, line 433
def test_retains_encoding
  key = "\xC3\xBCmlaut".force_encoding(Encoding::UTF_8)
  assert @cache.write(key, "1", :raw => true)
  assert_equal Encoding::UTF_8, key.encoding
end