Sivir's Spellshield vs. Tristana's E?

I'm not a sivir player, but from the viewpoints of a tristana player, I don't think it's fair that Sivir is able to spellshield Tristana's E at the very last tick of the bomb. Sivir already counters tristana enough, and during laning phase, it's extremely hard for me to do anything to try and poke her down with my E. I think it would be fair only to allow Sivir to use her spellshield right before the E lands on her, but we shouldn't allow Sivir to use her spellshield right before the bomb explodes.