I added the value Unknown to Type in CompoundValueNode.
So, 'Unknown' is now a valid value of the type, to be used when you want to create a node but you still don't know its type.

Now, before adding this valid constant ('Unknown'), I used the string "Unknown" in the function that converts constants of the type to ASCII strings.
I had to change that old "Unknown" and I picked "Not Known". Never liked it.
I will change "Not Known" for "Invalid". However, it is never used.