Issue with skeletal mob spawner

What I expected to happen was...:
Nothing.

What actually happened was...:
I got shot with an invisible arrow while near the top of my skeleton spawner and I fell down (while on peaceful!!).

Steps to reproduce...:
1. Find (or spawn) a skeleton spawner (and turn on peaceful)
2. Dig down to level 20 (or any other desirable height, not sure if height has to do with it)
3. Climb near the top of the spawner (not exactly on the spawner) with a cobblestone tower or any other method
4. See if the invisible arrow shoots you down

Note: This happened to me 2 times. Once I got lucky and I lived. But the next time I died cause of low health. Thanks for reading this.