XmlPullParser.getAttributeValue( )

I need some clarification on
XmlPullParser.getAttributeValue(java.lang.String namespace,
java.lang.String name). My schema is as follows:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema
  targetNamespace="http://www.user.org/user-schema"
  xmlns="http://www.user.org/user-schema"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified">

  <xsd:complexType name="User">
    <xsd:sequence>
      <xsd:attribute name="username" type="xsd:string"/>
      <xsd:attribute name="password" type="xsd:string"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:element name="user" type="User"/>
</xsd:schema>

My instance is as follows:

<user
  xmlns="http://www.user.org/user-schema"
  username="john" password="john"/>

I was expecting to get the username attribute as follows:

  parser.getAttributeValue("http://www.user.org/user-schema",
"username");

However xpp3 did not like the namespace parameter: I set it to null and
even that did not work. Finally it worked with the following change:

  parser.getAttributeValue("",  "username");

Is this the expected behavior? Why?

Thanks.
Naresh