The prefix '' cannot be redefined from '' to 'http://www.sitemaps.org/schemas/sitemap/0.9' within the same start element tag

I'm getting this error

The prefix '' cannot be redefined from '' to 'http://www.sitemaps.org/schemas/sitemap/0.9' within the same start element tag

while running this code

Dim writer As XmlWriter = XmlWriter.Create(FileLocation + "StaticUrls3.xml")
Dim urlList As New List(Of String)

urlList.Add("link1")
urlList.Add("link2")
urlList.Add("link3")       

writer.WriteStartDocument()
writer.WriteStartElement("urlset")
writer.WriteAttributeString("xmlns", "http://www.sitemaps.org/schemas/sitemap/0.9")

For Each aUrl As String In urlList
    writer.WriteStartElement("url")
    writer.WriteElementString("loc", aUrl)
    writer.WriteEndElement()
Next

writer.WriteEndElement()
writer.WriteEndDocument()
writer.Close()

Why am I getting this error?