Error importing XML through MGCB - "The XML element 'item' is required!"

[foxoftheasterisk]

I'm importing an XNA project into MonoGame, and getting the following error when trying to build my XML file:

Microsoft.Xna.Framework.Content.Pipeline.InvalidContentException: The Xml element Item is required!

This same file worked in XNA (and still does) so assuming I'm understanding MonoGame's mission correctly, it should also work in MonoGame.
Unfortunately I have little to no idea why it's not working. I expect the field most likely to be the issue is texture, which is an array of Color; other than that, I got nothing.

The XML file is as follows:

<?xml version="1.0" encoding="utf-8"?>
<XnaContent>
  <Asset Type="OutpostLibrary.Material[]">
    <Item>
      <materialName>dirt</materialName>
      <key>dirt</key>
      <transparency>opaque</transparency>
      <solidity>solid</solidity>
      <texture>FF886542 FF916B46 FF916B46 FF886542 FF9F764D FF886542 FF9F764D FF886542 FF916B46 FF916B46 FF916B46 FF9F764D FF886542 FF886542 FF9F764D FF916B46 FF886542 FF916B46 FF916B46 FF886542 FF886542 FF886542 FF916B46 FF9F764D FF9F764D FF9F764D FF886542 FF916B46 FF886542 FF886542 FF886542 FF916B46 FF9F764D FF916B46 FF886542 FF916B46 FF9F764D FF916B46 FF916B46 FF886542 FF886542 FF916B46 FF9F764D FF886542 FF886542 FF9F764D FF916B46 FF886542 FF916B46 FF916B46 FF916B46 FF9F764D FF886542 FF916B46 FF916B46 FF886542 FF9F764D FF916B46 FF9F764D FF9F764D FF916B46 FF916B46 FF9F764D FF916B46 FF916B46 FF916B46 FF886542 FF916B46 FF916B46 FF886542 FF9F764D FF916B46 FF886542 FF9F764D FF9F764D FF886542 FF886542 FF916B46 FF9F764D FF916B46 FF886542 FF9F764D FF886542 FF916B46 FF916B46 FF9F764D FF886542 FF916B46 FF886542 FF886542 FF886542 FF9F764D FF916B46 FF9F764D FF9F764D FF886542 FF916B46 FF9F764D FF886542 FF886542 FF916B46 FF916B46 FF9F764D FF886542 FF9F764D FF9F764D FF886542 FF916B46 FF886542 FF886542 FF916B46 FF916B46 FF9F764D FF886542 FF886542 FF886542 FF9F764D FF886542 FF916B46 FF9F764D FF916B46 FF886542 FF9F764D FF916B46 FF916B46</texture>
    </Item>
    <Item>
      <materialName>air</materialName>
      <key>air</key>
      <transparency>transparent</transparency>
      <solidity>gas</solidity>
      <texture>00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000</texture>
    </Item>
    <Item>
      <materialName>void</materialName>
      <key>none</key>
      <transparency>transparent</transparency>
      <solidity>vacuum</solidity>
      <texture>00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000</texture>
    </Item>
  </Asset>
</XnaContent>

And the definition of OutpostLibrary.Material:

    public class Material
    {
        public static Material[] matList;
        public static Dictionary<String, Material> materials;

        public String materialName;
        public String key;
        public Transparency transparency;
        public Solidity solidity;
        public Color[] texture;

        public Material()
        {
            texture = new Color[Navigation.VoxelsPerEdge * Navigation.VoxelsPerEdge * Navigation.VoxelsPerEdge];
        }

        public Color this[int i, int j, int k]
        {
            get
            {
                return texture[i * Navigation.VoxelsPerEdge * Navigation.VoxelsPerEdge + j * Navigation.VoxelsPerEdge + k];
            }
            set
            {
                texture[i * Navigation.VoxelsPerEdge * Navigation.VoxelsPerEdge + j * Navigation.VoxelsPerEdge + k] = value;
            }
        }
    }

    public enum Transparency { transparent, translucent, opaque }
    public enum Solidity { solid, liquid, gas, vacuum, plasma }

This might be, but probably isn't, related to issue #3404.

[tomspilman]

Thanks @Littlebeast . This should make it easy to build a unit test to figure out this bug.

[tgjones]

This is caused by the indexer, which is ignored by XNA but not by MG. I'm working on a PR now.

[tomspilman]

@Littlebeast - The fix was merged... let us now if you have more trouble with this.