Fall through slabs when exiting minecart

In the nether, we're making a transport system. The ground the rails are on is made of top-half slabs. I got into our minecart, and when I exit, I fell straight through the floor onto the ground 30 blocks below me.

What I expected to happen was...:
I thought I would exit the minecart and stay safe in our transport tunnel.

What actually happened was...:
I fell through the slab floor.

Steps to Reproduce:
1. Place a top-half slab in the air
2. place minecart rails on top
3. get in and exit the minecart