Why does output say "Unable to cast Instance to int64" in my mannequin script?

I'm making a shop where a player can click on a mannequin's clothes and get information on the clothes, buy the product, or add it to a cart to try on. I set up a RemoteFunction called OpenItemCard in the ReplicatedStorage and whenever the player clicks on clothes, I want the script to invoke OpenitemCard to send information from the mannequin to the client. Whenever I click on the mannequin all that shows up on output is "Unable to cast Instance to int64," and the cause of the error is at "Script 'Workspace.Mannequin.ShirtPurchase.ClickDetector.WhenClicked', Line 5," and "Script 'Players.BronzedMocha.PlayerGui.ItemCard.InfoLoadInScript', Line 15" What do?

server side script (line 5 mentioned above is line 5 below)

Adornee = script.Parent.Parent.Parent.AdorneeAnchor
ShirtID = string.sub(script.Parent.Parent.Parent.Body.Shirt.ShirtTemplate, 14)
script.Parent.MouseClick:connect(function(plr)
    print("Click Detected")
    game.ReplicatedStorage.OpenItemCard:InvokeClient(plr, ShirtID, nil, Adornee)
    print("Client Invoked")
end)

here's the remote function being called on the client side (line 15 mentioned above is line 15 below)

OpenItemCard = game.ReplicatedStorage:WaitForChild("OpenItemCard")

function OpenItemCard.OnClientInvoke(player,ShirtID,PantsID,Body)

    local function ShirtorPants()
        if ShirtID then
            return ShirtID
        else
            return PantsID
        end
    end 

    local Card = script.Parent.ItemInfo
    local CardHolder = script.Parent
    local Asset = game:GetService("MarketplaceService"):GetProductInfo(ShirtorPants())