Overriding functions in the default namespace

If I specify a namespace with a prefix, a custom function works fine:

from lxml import etree
_function_ns = etree.FunctionNamespace('http://example.com')
_function_ns.prefix = 'a'
def _dummy_count(arg):
 return 10
_function_ns['count'] = _dummy_count
_xml = etree.Element('structure')
print _xml.xpath('a:count()')

Output is '10.0'.

But if I add the function to the default namespace, I've got an error:

from lxml import etree
_function_ns = etree.FunctionNamespace(None)
def _dummy_count(arg):
 return 10
_function_ns['count'] = _dummy_count
_xml = etree.Element('structure')
print _xml.xpath('count()')

Output:

Traceback (most recent call last):
  File "/Users/user/Desktop/function-namespace.py", line 7, in <module>
    print _xml.xpath('count()')
  File "src/lxml/etree.pyx", line 1575, in lxml.etree._Element.xpath
  File "src/lxml/xpath.pxi", line 307, in lxml.etree.XPathElementEvaluator.__call__
  File "src/lxml/xpath.pxi", line 227, in lxml.etree._XPathEvaluatorBase._handle_result
lxml.etree.XPathEvalError: Invalid number of arguments

Seems it doesn't recognise my 'count(arg)' function and use standard 'count(...)' instead.

It works fine with custom functions that are not conflicting with standard functions, but how can I specify a custom function with one of the standard functions name?

---

Python : sys.version_info(major=2, minor=7, micro=15, releaselevel='final', serial=0)
lxml.etree : (4, 3, 0, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 32)
libxslt compiled : (1, 1, 32)