Player cannot respawn if slabs are above current bed

If you place a slab on the top half of the block space just above the bed, it will not set your spawn point. When you sleep in it, you still get the achievement "Sweet Dreams" and you are also forced into the "Swimming" animation due to the block height, but upon death and clicking respawn, it claims your home bed was missing or obstructed. If I can wake up on in the first time, why can't I spawn on in the next? I understand it's ubstructed, but due to our new player heights, I don't see why this should be an issue.