Sharpener keeps running even with no pencil inserted?

I have the opposite issue from a lot of people where their sharpeners won't run. Ours runs constantly, whether there's a pencil in it or not! To make it stop, we have to open the shavings drawer to release the switch that keeps the sharpener from operating when it's not there. I've tapped the sharpener vigorously and blown any shavings and debris out - everything I could see from outside the sharpener, down the bore hole and from what's accessible when the shavings drawer is removed, but it still runs continuously whenever connected to the wall plug.

I'm about to do an exploratory dissasembly, but I'm still wondering: what might be causing this? A problem with the insertion sensor? Is there anything well-known?

Thanks :)