Pick block button on mobs gives blank egg

I was playing around on the new snapshot when I tried "pick up" a mob spawner egg with Button 3. This gave me a blank egg. This happened on all mobs i tried (Rabbit, Mooshrom, Zombie, Chicken and Skeleton)
I am playing on a Mac Air

All the photos is after I pressed Button 3.

Video: http://youtu.be/qBucwPQlrZE