Upon typing /clear @p diamond 0 64
or /clear @p minecraft:diamond 0 64
In a command block or directly in chat.

Chat outputs 'Could not clear the inventory of Osky2918, no items to remove.

However if I use my IGN instead of @p like this:
/clear Osky2918 diamond 0 64
or /clear Osky2918 minecraft:diamond 0 64
It works perfectly fine.