If my suggestion to use 0 as End time or the same time for both Start and End doesn't work, I think you can use
6000 ms, since sewix has this value setted in his options:
Bug in subtitles
As an alternative, you can put in your plugin a Configuration window with a textbox in which the user can set the preferred value
P.S. Maybe if you do this way, also
sewix's problem will be fixed with your plugin :)