Not a valid AIRI or AIR file

Trying this: adt -package -keystore HelloWorld.p12 -storetype pkcs12 -target bundle HelloWorld.app HelloWorld.xml

Gives me the error:

File /Users/cameron/HelloWorld/HelloWorld.xml is not a valid AIRI or AIR file

And here is the contents of that XML file:

<?xml version="1.0" encoding="UTF-8"?>

<application xmlns="http://ns.adobe.com/air/application/13.0">

    <id>examples.html.HelloWorld</id>

    <versionNumber>0.1</versionNumber>

    <filename>HelloWorld</filename>

    <initialWindow>

  <title>Hello World</title>

  <content>HelloWorld.html</content>

  <visible>false</visible>

  <minimizable>true</minimizable>

  <maximizable>false</maximizable>

  <resizable>false</resizable>

  <width>800</width>

  <height>600</height>

  <systemChrome>none</systemChrome>

  <transparent>true</transparent>

    </initialWindow>

</application>

What's the issue?

Running adt -version gives:

13.0.0.83

so the version is correct... and the XML file looks okay to me. Any ideas?