There was a flaw in my code anyway: on return of OnSendChat(), I was checking for the return type without checking first that anything got returned, for I made the (wrong) assumption that since I required one return value via the lua_pcall() to OnSendChat(), a Lua error would occur if no value was returned... In fact, no, lua_pcall() does not trigger any error when less return values than requested are returned (which could easily be considered a Lua bug...); it simply truncates the stack from any value in excess to the requested number...
I made the proper checks in today's release, to avoid such crashes and properly issue an error.