tag0 namespace added for elements in default namespace

I'm trying to parse and modify a Maven's pom.xml using Groovy's XmlSlurper. My pom.xml declares the namespace xsi.

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" 
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
     http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>a-group-id</groupId>
<artifactId>an-artifact-id</artifactId>

My Groovy source is as follows:

import groovy.xml.XmlUtil
def pom = new XmlSlurper().parse('pom.xml')
   .declareNamespace('': 'http://maven.apache.org/POM/4.0.0',
      xsi: 'http://www.w3.org/2001/XMLSchema-instance')
//manipulate the pom
println XmlUtil.serialize(pom)

As you notice, I've declared the first namespace as empty. However in the output tag0 is added everywhere.

<?xml version="1.0" encoding="UTF-8"?>
<tag0:project xmlns:tag0="http://maven.apache.org/POM/4.0.0"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
      http://maven.apache.org/maven-v4_0_0.xsd">
<tag0:modelVersion>4.0.0</tag0:modelVersion>
<tag0:groupId>a-group-id</tag0:groupId>
<tag0:artifactId>an-artifact-id</tag0:artifactId>

How to avoid that?

For the moment my workaround is removing the tags manually:

println XmlUtil.serialize(pom).replaceAll('tag0:', '').replaceAll(':tag0', '')