Pick block doesn't work on Ender Dragon

Summary:
When using pick block on an Ender Dragon, the player isn't given a spawn egg. All other mobs have pick block (including for example, a Wither).

Steps to reproduce:

1. Spawn an Ender Dragon in Creative mode.
2. Press the pick block button while looking at Ender Dragon.

Observed results:
Nothing happens.

Expected results:
A spawn egg should be given.