xmlrpc default arguments not working

When I pass the first string to user.index using xml, nothing is returned, but the second string is good

<?xml version="1.0"?>
<methodCall>
<methodName>user.index</methodName>
<params>
</params></methodCall>

<?xml version="1.0"?>
<methodCall>
<methodName>user.index</methodName>
<params>
<param><value><int>0</int></value></param>
<param><value><string>*</string></value></param>
</params></methodCall>