The /replaceitem command dosn't put Elytra in the armor slot.

The /replaceitem command is buggy.

*What I expected to happen was the Elytra to be placed in my armor slot.

*What actually happened: after setting the command,
the command block gave me an error message.
PS. the command still works in other slots

Steps to Reproduce:
1. Place a command block down an put in the following command.
2. /replaceitem entity [name] slot.armor.chest minecraft:elytra 1 0

{Unbreakable:1}

3. Activate it (I've already tried all the different ways)