Comparators do not detect when a single item passes through a hopper

What I expected to happen was...:
Comparator detects the item and gives a short pulse

What actually happened was...:
Nothing happened

Steps to Reproduce:
1. Place 2 hoppers, one on top of the other
2. Place a comparator adjacent to the top hopper
3. Throw an item into/ place an item into the top hopper