You can't use the execute command with witherskulls

When I summon a wither skull using the summon command, i use the execute command to summon primed tnt at the wither skull it does not work. I used
/execute @e[name=WitherSkull] ~ ~ ~ summon PrimedTnt ~ ~ ~ {Fuse:0}
/summon WitherSkull ~ ~ ~ {ExplosionPower:0,direction:[0.0,0.0,0.0]}
/setblock ~1 ~ ~ redstone_block 0 destroy

I tested it with the fishing bob
/execute @e[name=unknown] ~ ~ ~ summon PrimedTnt ~ ~ ~ {Fuse:0}

When i place the execute command under a really fast redstone clock which is the set block it says unknown command