Shears in dispensors are unbreakable

I built a automatic sheep farm in creative mode. But the shears in the dispensors are unbreakable. one of them has a durability from -54 as seen on the screenshot. even when i take it out of the dispensor and put it back in it doesn't brake.

What I expected to happen was...:
the shears break

What actually happened was...:
the shears have a durability of -1 or below

Steps to Reproduce:
1. Put a shear in a dispensor
2. Shear sheeps until the shear should break
3. The shear doesn't brake