Some call me Tim |
Well, the rules-as-written say to use your "Charisma bonus" not "Charisma modifier." So, strictly speaking a negative Charisma modifier would not be added to this roll.
Based on the wording about "This ability functions just like a Diplomacy check," I don't think that was their intent and that the subtle difference between bonus and modifier slipped past the editor.
Alas, dura lex sed lex.