I built my first nether portal in a world, and it opened under lava

What I expected to happen was...:
I expected it to open with at least some air so I wouldn't immediately die.

What actually happened was...:
When the portal animation completed, the screen was red. I could not move, and my hearts gradually ticked away until I died.

After reappearing at my spawn point in the overworld, I went back to the portal and entered it again. This time, when the portal animation was complete, the screen was black. Again the health drained away quickly, and the message said I died within rock.

Steps to Reproduce:
1. I created a world and let it generate a random seed in 14w25a
2. I upgraded to 14w25b.
3. I played for several hours over several days with this version
4. I created a portal at altitude y=67, with an opening 2 wide by 3 high (i.e. the frame was 4 wide by 5 high)
5. I stepped into it and let the animation complete