ProcessReceipt is returning nil values?

I will show you the script I have written for a Speed potion in a game of mine.

local marketService = game:GetService("MarketplaceService")
marketService.ProcessReceipt = function(purchaseId, playerId, productId)
    print(purchaseId)
    print(playerId)
    print(productId)

    return Enum.ProductPurchaseDecision.PurchaseGranted
end

"purchaseId" had a value, which I will not tell the value of due to the uncertainty of safety from revealing its value. "playerId" and "productId", however, have the value of nil. What may be causing this? I have tried researching on this website, and have remained dumbfounded as to what the problem is. Please, give me a solution ASAP.