Unity C# XML Serialization broke when abstract base type is added

I had a menu system with 4 different item types instantiated from one XML file. I had always intended to build an abstract base class for them to inherit from, after I knew what the common elements would be. When I wrote the simple base class, and assigned the 4 existing items as derived classes, everything broke! The GameObjects with my instantiator scrips kick this error, now:

InvalidOperationException: To be XML serializable, types which inherit from IEnumerable must have an implementation of Add(System.Object) at all levels of their inheritance hierarchy. UnityEngine.Transform does not implement Add(System.Object) 

I read a lot about why it could happen, but like I said, it wasn’t happening until I build a new, very simple abstract class for the items to inherit from.
Any help understanding “why” would be awesome!