Can not use /particle for the bobber of a fishing rod (@e[name="unknown])

You can not 'spawn' paticles with the following command at a bobber of a fishing rod:
/particle dripWater ~ ~5 ~ 3 0.1 3 0.001 500 @e[name=unknown]
It says: "That Entity cannot be found."

The command works on named villagers, and even when I only do:
/particle dripWater ~ ~5 ~ 3 0.1 3 0.001 500 @e
It does spawn the particles above every entitys head, except the bobber.
It also say, after all the "Playing effect dripWater for 500 times" The same: "That Entity cannot be found"

But when I run the /tp command like this:
/tp @e[name=unknown]
It teleports me to the bobber.