The replaceitem entity command does not work with shields on mobs

Whenever I try to put shields on mobs using the replaceitem entity command the shield simply disappears. For example I was trying to create a zombie fully armored with iron armor and a shield on his offhand but instead the shield disappears when I use the command. The zombie comes out fully armored but the shield isn’t there.

This is the command. /replaceitem entity @e[type=zombie] slot.weapon.offhand 0 shield