XPath function `string-length` without argument not working

[tompng]

# Expected
Nokogiri::XML.parse('<root><item>123</item><item>1234</item><item>12345</item></root>').xpath('//*[string-length()=4]')
# => [#<Nokogiri::XML::Element:0x1450 name="item" children=[#<Nokogiri::XML::Text:0x1448 "1234">]>]

# Actual
REXML::XPath.match(REXML::Document.new('<root><item>123</item><item>1234</item><item>12345</item></root>'), '//*[string-length()=4]')
# => 'REXML::Functions.string_length': wrong number of arguments (given 0, expected 1) (ArgumentError)

REXML::VERSION
# => "3.4.4"

string-length with argument seems to be working as expected

Nokogiri::XML.parse('<root/>').xpath('string-length("hello")')
# => 5.0

REXML::XPath.match(REXML::Document.new('<root/>'), 'string-length("hello")')
# => [5]