I realized after I got the same error you did that I had to modify the diceString property. The first edit was the old version while the 2nd edit is the new version. I know have included open ended dice rolls in my RollDiceBase Class which can be seen in Generic System form.
I will also change the above property so that it will not be dangerous.
EDIT 2 : I have updated the soure using your suggestion about the propertynames and also add some documentation about the open ended function in the RollDiceBase Class.