Shop Function Error

[Donut]

I have this function for a shop but its always saying the player doesnt have the items needed even when they do

function test(p: player, item: item, cost: item):
    if {_p}'s inventory contains {_cost}:  
        remove {_cost} from {_p}'s inventory 
        give {_p} {_item}
    else:
        broadcast "nope"

command /test:
    trigger:
        test(player, 5 diamonds, 5 dirt)

i have no idea why that doesnt work because same exact thing works as a command:

command /test <player> <item> <item>:
    trigger:
        if arg-1's inventory contains arg-3:  
            remove arg-3 from arg-1's inventory 
            give arg-1 arg-2
        else:
            broadcast "nope"

im running spigot 1.12 and skript dev29