Impaling on drowned doesn't work

[rocky crystal]

            spawn 1 of drowned at {var}
            set {_item} to leather cap of mending
            set {_item1} to leather tunic of mending
            set {_item2} to leather pants of mending
            set {_item3} to leather boots of mending
            set {_item4} to trident of impaling 5
            dye {_item} (0, 25, 25)
            dye {_item1} (0, 0, 25)
            dye {_item2} (0, 25, 25)
            dye {_item3} (0, 0, 25)
            add unbreakable and mending 1 to {_item}
            add unbreakable and mending 1 to {_item1}
            add unbreakable and mending 1 to {_item2}
            add unbreakable and mending 1 to {_item3}
            add unbreakable and mending 1 to {_item4}
            equip last spawned drowned with {_item} 
            equip last spawned drowned with {_item1}
            equip last spawned drowned with {_item2}
            equip last spawned drowned with {_item3} 
            set last spawned drowned's held item to {_item4}
            set last spawned drowned's max health to 110
            set last spawned drowned's health to 70
            set display name of last spawned drowned to "&c&lZombie Knight"

[storm storm]

impaling only does extra damage on each hit against aquatic mobs

[rocky crystal]

oh

so how can i do it

[storm storm]

idk