Kill command doesn't work for signs independently

I'm trying to kill sign item drops, but I don't want to kill all item drops. /kill @e[type=item] works to kill all items including signs, and /kill @e[type=item,name=sand] works to kill only sand drops, same with other blocks, but signs do not. I've tried sign, wood_sign, wooden_sign, wall_sign, standing_sign, & oak_sign but no luck. When you kill all items, it lists off oak sign as one of the killed items if it's a drop at the time.