Hmmm. Good question.
Technically a Shi result is a 0 and therefore the lowest die.
But if you use them to also bring bad stuff into the game, expertise would allow you to avoid that in most cases.
Although, you are meant to be good at the skill so is that a problem?
So I'd offer two answers.
If it is just a 0 in your game, then it is certainly the lowest and can be rerolled.
But if the GM is using them to bring the attention of the spirits to the table,
I'd rule they can't be rerolled with just Expertise.
You might insist that only at a certain skill level can Expertise be used to reroll a Shi.
Really one for the individual GM and how you want to play the game I suppose.