Command block cannot use /summon under any circumstances.

Command blocks cannot /summon any entity.

What I expected to happen was...:
It would summon the specified object.

What actually happened was...:
Nothing happened and the command block output said 'Unable to summon object.'

Steps to Reproduce:
1. Get a command block.
2. Input a '/summon' command using any entity.
3. Activate it using redstone.