I don't think we can fix this. The parameter must really be a constant - as in a string literal, not even a global constant. The problem if we allowed this is you could make an event referencing a global constant, run the game, then later at some point decide you don't want that global to be a constant any more, so you edit it back to be a normal variable and add some events to change it at runtime. Then the event you previously made is no longer valid.
What's the benefit of using a global constant anyway? It doesn't seem like it saves you any typing or improves maintenance at all.