Error with /clearing an item from armor and shield slots

Error with /clearing an item from armor and shield slots.
The normal /clear command should remove items from the armor and shield slots, like the other inventory slots.
Instead, I get the message "An unknown error occurred while attempting to perform this command"
Steps to Reproduce:
1. Put any item in your armor or shield slots.
2. Type /clear into your chat and press enter.
3. Read the message.