[CanEditMultipleObjects] + Custom Editor NOT working

I have a custom editor that can edit any Asset which is a child of class ArmourAsset

ArmourAsset has 4 children, HelmAsset, ChestAsset, RingAsset, LegsAsset.

The custom editor has Can edit multiple objects - but it ONLY runs the editor if I select multiple objects of the *same* asset type. My editor uses reflection to properly update the values of each ArmourAsset and doesn't care about the child type - but the editor refuses to display the custom inspector I've written.

How do I fix this?

 

Example code:


[CanEditMultipleObjects]
[CustomEditor( typeof( ArmourDataAsset ), true, isFallback = false )]
public class ArmourDataAssetEditor : UnityEditor.Editor
{
public override void OnInspectorGUI() {
// Correctly handles multiple object types as long as they are armour data assets or their children.
}
}
Why won't unity display the inspector? Instead it says 'Narrow the selection'. This is completely irritating.